Enhance whitespace marker support and refactor indent guide handling

[mkslanc]

Issue #, if available:

Description of changes:

• Add support for multiple whitespace markers in selection
• Add pre-rendered whitespace glyphs when "Whitespaces in selection" extension is on to prevent layout shift on varwidth fonts

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Pull Request Checklist:

• No backwards incompatible changes were made to Ace's public interface which is defined through the main typings file (ace.d.ts) and its references:
  • https://github.com/ajaxorg/ace/blob/master/ace.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-modes.d.ts
  • https://github.com/ajaxorg/ace/blob/master/ace-extensions.d.ts

Open kitchen-sink @ ada20b74159445c75d29f48ba0b83251d816ff5a