Fixes #6878: Add limits to the ammount of text we write to the <textarea> for screen readers to avoid stalling the UI.

markdown mode uses TextualSuggestSupport, remove used id field, remove unused service dependencies, also fixes #6778

this new setting, which defaults to false, allows to keep peek editors on  double click and ESC. This is useful for serious coding inside peek windows

Fixes #6628: Add `isBuiltin` flag to language feature registration to get the previous sorting/behaviour for builtin language supports