feat: improvements to loading project-local settings

[mrcjkb]

• load settings from .vscode/settings.json (Closes More flexibility in root_dir selection inside a workspace for large monorepo #286)
• Improvements to overriding behaviour when loading rusta-analyzer.json (Need help getting leptosfmt to work with rust-analyzer  #283)

@GuillaumeLagrange this implements support for loading settings from .vscode/settings.json.
You can test it with the vscode-settings-json branch.

I played around with workspace/didChangeConfiguration briefly, but couldn't get it to reload the configuration without behaving buggy.
If you come up with something that works, please let me know 😄

Note: Because the behaviour could be perceived as buggy, I have disabled it by default.
You can enable it with vim.g.rustaceanvim.server.load_vscode_settings = true

[github-actions]

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• Tested
  • Tests have been added.
  • Tested manually (Steps to reproduce in PR description).
• Updated documentation.
• Updated CHANGELOG.md

[GuillaumeLagrange]

@mrcjkb it works like a charm, thank you very much.

I'll look into dynamically reloading with workspaceDidChangeConfiguration and see if I can come up with anything, but as it is I can definitely ditch vs code again.
You might hear more from me regarding huge repo behavior in the future, thanks again !