Separate and expand colour schemes from preferences

[abrudz]

Describe the issue you are having

Store each custom colour scheme as a separate text file with a name matching (and defining) the scheme name. For example

"[{\"name\":\"My Look\",\"theme\":\"dark\",\"styles\":\"asgn=fg:#50fa7b com=fg:#6272a4 diam=fg:#ff79c6 err=fg:#ff5555,bgo:1 fn=fg:#8be9fd idm=U kw=fg:#ff79c6 lnum=bgo:0,fg:6272a4 mtch=bgo:0.5,fg:#f8f8f2,bg:#44475a …

should become

• On Windows: %HOMEPATH%\ride\46\ColSchemes\My Look.json
• On MacOS: $HOME/Library/Preferences/ride/46/ColSchemes/My Look.json
• On Linux: $XDG_CONFIG_HOME/ride/46/ColSchemes/My Look.json
  If $XDG_CONFIG_HOME isn't set, default to $HOME/.config

and contain:

{
  "chrome": "dark",
  "styles": {
    "asgn": {
      "fg": "#50fa7b"
    },
    "com":  {
      "fg": "#6272a4"
    },
    "diam":  {
      "fg": ":#ff79c6"
    },
    "err":  {
      "fg": "#ff5555",
      "bgo": 1
    },
    "fn":  {
      "fg": "#8be9fd"
    },
    "idm": {
      "U": true
    },
    "kw":  {
      "fg": "#ff79c6"
    },
    "lnum": {
      "bgo": 0,
      "fg": "#6272a4"
    }
    "mtch": {
      "bgo": 0.5,
      "fg": "#f8f8f2",
      "bg": "#44475a"
    }
…

Notes:

• I've renamed "theme" to "chrome" to lessen confusion and match the UI.
• B, I, and U are currently not name:value pairs; they become "B": true etc.
• # can currently be omitted from hex codes; that should not be allowed. Instead, we can allow additional CSS colour specifications, e.g. rgb(10,20,30)