feat: add "enabled supported languages" setting

This adds a new setting called `Enabled Supported Languages`. This
presents to the user as checklist of languages, which are all enabled by
default.

In reality, it is a denylist. Only languages which are unchecked have
any effect on Code Suggestions. This is true in both states of the
`gitlab.featureFlags.languageServer`.

See https://gitlab.com/gitlab-org/editor-extensions/gitlab-lsp/-/issues/174
for the rationale.

Part of #1388.