Introduce .ruby-lsp.yml config file

[andyw8]

Motivation

We want to have a general purpose configuration mechanism for Ruby LSP that not tied to a particular editor.

Also closes #1295 (but with a slightly different approach)

Implementation

As agreed with @vinistock, we will repurpose the current indexing configuration file, .index.yml.

There will be a new top-level key, indexing, but within that the structure was the same as before.

Automated Tests

Included

Manual Tests

• Check out this branch
• Ensure indexing is working (e.g. the Jump to Definition works for constants)
• Edit .ruby-lsp.yml and rename the indexing key.
• Restart the LSP and verify that error notification appears (the remaining LSP feature should continue working)
• Copy .index.yml from main, and delete .ruby-lsp.yml.
• Restart the LSP and ensure a deprecation warning about .index.yml shows in the output panel.
• Ensure an error message is showing but otherwise everything works (e.g. formatting)
• Delete .index.yml and restart the LSP (so now there are no config files)
• Ensure indexing completes successfully (e.g. the Jump to Definition works for constants)

[andyw8]

I'm wondering if this should display a notification.

[andyw8]

(looks like some tests need a slightl adjustment for Windows)

[vinistock]

I think the changes are good, but I wonder if having the configuration file isn't deviating from the LSP approach. We might be better off getting rid of the .index.yml (with a deprecation warning first) and then start accepting all of the configuration from the editor.

After #1818 is shipped, we could have the indexing configuration be a part of the global state and gather the configuration from the initialization options.

I think it's worth ensuring we're following the LSP approach at this point, because otherwise we will have introduced two configuration files (.index.yml and .ruby-lsp.yml), both of which may be not following the standard.

Thoughts?

[andyw8]

TODO: update ruby-lsp-doctor to handle new config