Enable support for an LSP client to open source generated files

[jasonmalinowski]

This allows source-generated documents to be opened by an LSP client. A custom URI format is defined which uniquely defines a source generated document by serializing the SourceGeneratedDocumentIdentity; this URI is returned any time we need to navigate to a document or reference the document in any way. A custom request is also defined to fetch the text of a document which can be used for the client; our VS Code extension will implement a TextDocumentContentProvider to call that.

[CyrusNajmabadi]

Looking good overall. Def would like to understand some parts a little better. Feel free to include info here, or just ping me directly :)

[jacobjmarks]

Bump on this; I believe @jasonmalinowski has gone on leave. Are we able to reassign?