Add bracket colors plugin

[asifamin13]

Bracketcolors plugin

• Color { }, [ ], ( ) based on nesting order to make it easier to see the start/end bracket
• Ignore brackets in non source code (strings, comments, doc strings, etc.)
• Language agnostic

How it works

Allocates 3 indicators (starting at INDICATOR_IME - 3) to color each bracket pair as determined by SCI_BRACEMATCH. Ignore non source brackets as determined by highlighting_is_code_style

Dependencies

• Geany 1.38
  • Need INDICATOR_IME from updated scintilla release
• C++17
  • Mainly for std::map

Demo

[asifamin13]

This is revived from my original pull request

I got too aggressive squashing commits between my work and home workstations and ended up getting other unrelated commits from upstream into my feature branch 😅

[asifamin13]

cc @elextr @eht16 @b4n @rdipardo