feat: add node for Python function definitions (#157)

* Add `function_definition` node for Python

Using parameters as target. Inspired by the same functionality in
`r.lua`.

* Add test specs for python `function_definition`

Re-using the same code as for arguments

* Add comment for `function_definition` in Python test `index.py`

lua/treesj/langs/python.lua

@@ -115,4 +115,7 @@ return {
       'argument_list',
     },
   },
+  function_definition = {
+    target_nodes = { 'parameters' },
+  },
 }

tests/langs/python/python_join_spec.lua

@@ -58,6 +58,15 @@ local data_for_join = {
     expected = { 48, 49 },
     result = { 54, 55 },
   },
+  {
+    path = PATH,
+    mode = 'join',
+    lang = LANG,
+    desc = 'lang "%s", node "function_definition", preset default',
+    cursor = { 55, 4 },
+    expected = { 48, 49 },
+    result = { 54, 55 },
+  },
   {
     path = PATH,
     mode = 'join',

tests/langs/python/python_split_spec.lua

@@ -58,6 +58,15 @@ local data_for_split = {
     expected = { 54, 58 },
     result = { 48, 52 },
   },
+  {
+    path = PATH,
+    mode = 'split',
+    lang = LANG,
+    desc = 'lang "%s", node "function_deinition", preset default',
+    cursor = { 49, 4 },
+    expected = { 54, 58 },
+    result = { 48, 52 },
+  },
   {
     path = PATH,
     mode = 'split',

tests/sample/index.py

@@ -45,13 +45,13 @@
     end="!"
 )
 
-# RESULT OF JOIN (node "parameters", preset default)
+# RESULT OF JOIN (node "parameters" and "function_definition", preset default)
 def create_adder(x, b):
     def adder(y):
         return x + y
     return adder
 
-# RESULT OF SPLIT (node "parameters", preset default)
+# RESULT OF SPLIT (node "parameters" and "function_definition", preset default)
 def create_adder(
     x,
     b