Add support for resolving documentation links to local documentation

[zacps]

This will consist of:

• Add the actual logic for resolving links (and bare definitions for the open docs command) to local documentation
• Add a clientside trampoline so that file:// URLs can be opened with a browser instead of in the editor

[zacps]

To expand on the trampoline:

Currently if we include a file:// URL in a hover vscode will open the file when it's clicked on and we can't change that. Viewing the generated HTML docs in the editor is almost certainly not intended.

Instead we'll generate the local URL as normal on the server, then the client will rewrite it to a command URL encoding the actual filepath as JSON.

That will trigger a command on the client which can unpack the JSON to get the URL and then use the system's default application (hopefully a browser) to open the path.

Messy, but it should work.

Meanwhile, other clients which open file URLs outside of the editor by default need to do none of this work as they'll just get the normal URL.