test: add end-to-end tests for the various formatting requests

[alcarney]

Description (e.g. "Related to ...", etc.)

Following on from #440 this adds an example formatting.py server that implements the following requests

• textDocument/formatting
• textDocument/rangeFormatting
• textDocument/onTypeFormatting

The server is intended to help with formatting simple markdown like tables

Screencast_20240425_182444.webm

As the above clip shows the on-type formatting is a little clunky to use (and buggy in some scenarios) but it should be enough to serve as a demo on how to get started.
Also the relevant tests in tests/lsp have been migrated to end-to-end tests using the new example server.

Code review checklist (for code reviewer to complete)

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Commit messages are meaningful (see this for details)
• Tests have been included and/or updated, as appropriate
• Docstrings have been included and/or updated, as appropriate
• Standalone docs have been updated accordingly

[tombh]

🍎🍌

[tombh]

Wow, I'd never thought of using formatting for those kind of little things. LSP's uses are endless.