Handle embedded-content

[msaraiva]

Related to #278.

Currently, vscode-elixir-ls handles all template languages without parsing them, which brings a problem that all services, including code completion, hover, go to definition, etc. don't work as expected. Ideally, each template-specific extension should handle their own template and then forward to other services when required, for instance, when inside <style>, <script> or elixir expressions inside { } or <% %>.

This PR adds the "embedded-content" scheme to the list of documentSelector for all languages handled by this extension. This allows any other extension to properly forward any Elixir-related command to be handled by ElixirLS only when actually necessary.

Ideally, this extension (or the next official one) should only handle Elixir language. This way it can work consistently, without having to parse each individual template language. All other templates that have their own syntax should be responsible for handling their own LS requests, even if they, eventually, have to forward the request back to this extension.

A couple of examples of inconsistent code completions, listing Elixir-related suggestions where it shouldn't:

Inside <style>

Inside <script>

Inside HTML markup

By properly dealing with embedded-content, other extensions can handle their own templates, detect other embedded languages and then forward to the proper service. The service is handle by whatever extension the user has installed that is registered to handle the content type.

Here's an example using the Surface extension, which avoids bringing all the suggestions from ElixirLS, even when it makes no sense:

Listing only applicable components and HTML tags

Forwarding to Elixir service when inside expressions

Forwarding to CSS/Tailwind service when inside <style>

The vscode-elixir-ls should do the same when working with embedded templates using ~H or ~F. But that should be addressed on a separate PR.

[msaraiva]

"html-eex" and "phoenix-heex" should eventually be moved here.

[lukaszsamson]

Nice @msaraiva. One question: have you checked if ElixirLS handles embedded-content schema URIs correctly? I believe at least some of the providers should work as it essentially is similar to untitled schema

[msaraiva]

Hey @lukaszsamson!

Thanks for reviewing it.

As far as I could test, they all work just fine with embedded-content. I have only tried completions, hover and definitions, so far, but I assume it will work nice with the others. The vscode in those screenshots, for instance, were all using ElixirLS. I believe we could even use untitled if we wanted but maybe it's better to keep the scheme explicit, specially because anyone trying to integrate with our extension following the official Request Forwarding tutorial, would probably use embedded-content as specified there.