[Draft] Deeper integration with Agda: foundational work

[nvarner]

Draft PR for #35. Currently likely to break on all but simple files.

Most of the big changes are behind-the-scenes work to enable future development.

• Data model, which tracks the server's knowledge of AgdaFiles and AgdaLibs
• Indexer, which turns an Agda AST into an AgdaFile
• LSP handler infrastructure, which gives handler implementations access to the relevant AgdaFile
• Document symbol handler implementation for demonstration, which turns an AgdaFile into the "Outline" tab in VS Code

TODO:

• Rebase
• Major errors to identify and iron out (i.e. shouldn't break on agda-stdlib)
• Control via configuration/disable by default?