Move on_config hooks from providers to listeners

Fits a bit better and there may be other client generated events in the
future.

doc/dap.txt

@@ -1241,6 +1241,54 @@ For events, the listeners are called with two arguments:
 2. The event payload
 
 
+==============================================================================
+BUILT-IN CLIENT EVENTS                                     *dap-listeners-ext*
+
+In addition to the debug adapter protocol messages, the `dap.listeners` also
+provides hooks to listen on and react to synthethic events created by the
+client.
+
+Currently there are:
+
+- `dap.listeners.on_config` (See |dap-listeners-on_config|)
+
+==============================================================================
+ON_CONFIG EXTENSIONS API                              *dap-listeners-on_config*
+
+
+Plugins can pre-process the |dap-configuration| whenever a debug session
+starts.
+
+To do so, register a `on_config` hook in the `dap.listeners.on_config` table.
+
+The key for the table is a `plugin-id`. Plugins should use their plugin name.
+Do _not_ use the `dap.` namespace. It is reserved for nvim-dap itself.
+
+The value for the table is a function that takes a |dap-configuration| as
+parameter and must return a |dap-configuration|.
+
+Before making modifications to the config you should copy it, to ensure you
+don't make permanent changes to a configuration stored within
+`dap.configrations.<filetype>` via mutations.
+
+Example:
+>lua
+  local dap = require("dap")
+  dap.listeners.on_config["dummy-noop"] = function(config)
+    return vim.deepcopy(config)
+  end
+<
+To support async operations, the on_config functions are called within a
+coroutine.
+
+This functionality should only be used by plugins that implement generic
+functionality applicable for all or most configurations.
+If you are writing a plugin that already owns an adapter definition, you
+should prefer using the `enrich_config` function available in |dap-adapter|.
+If you are not owning the adapter definition but the plugin's functionality is
+still specific to a limited selection of adapters, please make sure you're not
+messing with configurations used with adapters foreign to your plugin.
+
 ==============================================================================
 PROVIDERS EXTENSIONS API                                       *dap-providers*
 
@@ -1291,39 +1339,6 @@ An example:
 To support async operations, the config providers functions are called
 within a coroutine.
 
-==============================================================================
-ON_CONFIG EXTENSIONS API                              *dap-providers-on_config*
-
-Plugins can pre-process the |dap-configuration| whenever a debug session
-starts.
-
-To do so, register a `on_config` hook in the `dap.providers.on_config` table.
-
-The key for the table is a `plugin-id`. Plugins should use their plugin name.
-Do _not_ use the `dap.` namespace. It is reserved for nvim-dap itself.
-
-The value for the table is a function that takes a |dap-configuration| as
-parameter and must return a |dap-configuration|.
-
-Before making modifications to the config you should copy it, to ensure you
-don't make permanent changes to a configuration stored within
-`dap.configrations.<filetype>` via mutations.
-
-Example:
->lua
-  local dap = require("dap")
-  dap.providers.on_config["dummy-noop"] = function(config)
-    return vim.deepcopy(config)
-  end
-<
-To support async operations, the on_config functions are called within a
-coroutine.
-
-This functionality should only be used by plugins that implement generic
-functionality applicable for all or most configurations.
-For configuration pre-processing that is specific to a individual adapter you
-should instead use the `enrich_config` function available in |dap-adapter|.
-
 ==============================================================================
 UTILS API                                                          *dap-utils*
 

lua/dap.lua

@@ -118,14 +118,17 @@ M.listeners = {
       rawset(tbl, key, {})
       return rawget(tbl, key)
     end
-  });
+  }),
   ---@type dap.listeners
   after = setmetatable({}, {
     __index = function(tbl, key)
       rawset(tbl, key, {})
       return rawget(tbl, key)
     end
-  });
+  }),
+
+  ---@type table<string, fun(config: dap.Configuration):dap.Configuration>
+  on_config = {}
 }
 
 
@@ -268,9 +271,6 @@ M.configurations = {}
 local providers = {
   ---@type table<string, fun(bufnr: integer): dap.Configuration[]>
   configs = {},
-
-  ---@type table<string, fun(config: dap.Configuration):dap.Configuration>
-  on_config = {},
 }
 do
   local providers_mt = {
@@ -388,7 +388,7 @@ do
     return ret
   end
 
-  providers.on_config["dap.expand_variable"] = function(config)
+  M.listeners.on_config["dap.expand_variable"] = function(config)
     return vim.tbl_map(expand_config_variables, config)
   end
 end
@@ -547,7 +547,7 @@ local function prepare_config(config)
     config = config()
     assert(config and type(config) == "table", "config metatable __call must return a config table")
   end
-  for _, on_config in pairs(providers.on_config) do
+  for _, on_config in pairs(M.listeners.on_config) do
     config = on_config(config)
   end
   return config