Folding Strategy: Brackets

partially continuation of #264780

• #264780
• https://github.com/expert-lsp/vscode-expert/issues/45

Currently VSCode's editor.foldingStrategy supports

1. auto - Use a language-specific folding strategy if available, else the indentation-based one.
2. indentation - Use the indentation-based folding strategy.

I propose a 3rd option - brackets this would use VSCode's new bracket pair colourizer to supply the folding ranges https://code.visualstudio.com/updates/v1_60#_high-performance-bracket-pair-colorization the bracket pair colourizer is quite performant, already built-in and correctly handles TextMate standard token types (which the current whitespace indention strat, language-configuration.json indentationRules, and often language extensions don't support embedded TextMate grammars)

and maybe a 4th option for brackets & indentation?