Migrate indexing configuration to editor settings

[vinistock]

Motivation

Closes #2156

Deprecate our .index.yml file and move to relying on editor settings instead. Editor settings are better suited for configuring language servers, rather than using custom configuration files.

Editor settings already handle different levels of hierarchy. For example, workspace settings and user settings - merging when appropriate. In addition to that, we can show descriptions and examples of settings, which provide a much more intuitive experience. Overall, a custom configuration file is just a worse experience for the users.

A small caveat is that, because editor settings are usually camel case and we use snake case in Ruby, we need to accept both. This is what the settings look like now:

{
  "excludedPatterns": ["**/test/**.rb"],
  "includedPatterns": ["**/bin/**"],
  "excludedGems": ["rubocop", "rubocop-performance"],
  "includedPatterns": ["rake"],
  "excludedMagicComments": ["compiled:true"]
}

Implementation

1. Started showing a warning for projects that are using the deprecated .index.yml
2. Started accepting the indexing configuration from initializationOptions
3. Added the new settings to the VS Code extension

Automated Tests

Changed some tests.

[andyw8]

Should the docs about .index.yml in the README have been updated yet? (I can open a PR if so).

[vinistock]

@andyw8 you're right, I totally forgot about the docs. If you can, please open a PR updating it!