We currently have a very unstructured approach to settings related to experimental features. A few examples:

• editor.experimental.treeSitterTelemetry
• editor.experimentalWhitespaceRendering
• accessibility.signalOptions.experimental.delays.warningAtPosition
• C_Cpp.experimentalFeatures
• timeline.pageOnScroll, its description starts with Experimental.

This is problematic in a few key ways:

• There's no deterministic way to discover all experimental settings
• There's no clear way of knowing whether a specific setting is experimental
• There's no easy way to mark a setting as non-experimental; one must migrate

We should:

1. Support 1st party labeling of settings to be experimental
2. Surface this labeling in the settings UI
3. Support an easy @experimental query in the settings UI

Optionally:

1. Migrate all existing experimental settings to the new infrastructure
2. Add eslint rules preventing from using *experiment* in the setting name