This PR adds the ability for extensions to implement
`language_server_workspace_configuration` to provide workspace
configuration to the language server.

We've used the Dart extension as a motivating example for this, pulling
it out into an extension in the process.

Release Notes:

- Removed built-in support for Dart, in favor of making it available as
an extension. The Dart extension will be suggested for download when you
open a `.dart` file.

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Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>