Allow configuring linters separate from formatter

[vinistock]

Motivation

Closes #1924

Since addons will now be able to register more linters, we need to account for the fact that users may want to lint with multiple tools that are different from their formatter. I suggest we allow configuring an array of linter identifiers, so that we'll gather diagnostics from all of those.

Note: when it comes to auto-formatting, I do not believe we should use all linters for it. I think it's best to follow the example of the TypeScript and Rust community, where linting errors are fixable via code actions, but not through formatting. Users can then configure to have code actions fixed on save with

{
  "[ruby]": {
    "editor.codeActionsOnSave": {
      "source.fixAll": "explicit"
    }
  }
}

Implementation

I split by commit.

1. Started remembering the configured linters
2. Started using all of them for diagnostics
3. Added the setting to the VS Code extension

Automated Tests

Adjusted our tests.