feat: language server toolkit

[lukealvoeiro]

Description

This PR creates the language server toolkit. This toolkit is used to stand up & communicate with language servers over the language server protocol. It draws heavy inspiration from https://github.com/microsoft/multilspy.

Remaining work

The following work will not be implemented in this PR, but is required before we make language servers part of the default:

• Prompt engineer a way to reliably convert searches for a symbol into requests to the LSP (instead of current ripgrep).
• (Blocked by above) Determine whether this is a net improvement over the current implementation.
• Tune the number of surrounding lines shown in the response from the language server.
• (Potentially) Use request_document_symbols to better inform context (though there is some evidence from Aider that this doesn't scale well on large repos).
• Implement the TODO in session.py@line:189, to update the language server about any files that may have been changed by the user while goose is running.