For #271167
This makes it so our built-in extensions can mostly be built using `tsc` on the command line. Previously the extensions were picking up a lot of typing info from the root `node_modules` that meant they weren't truly independent
* Contribute to json language server with a custom language.
* Add `snippets` to `"activationEvents"`
* Remove hardcoded `snippets` from `documentSettings`
* Fix wrong variable in `!isEqualSet()`
* Use `extensions.allAcrossExtensionHosts` instead of `extensions.all`
* enable `"enabledApiProposals"` for `extensions.allAcrossExtensionHosts`
* Fix error: `Property 'allAcrossExtensionHosts' does not exist on type 'typeof extensions'`
* Remove `snippets`