fix(formatter): don't let overrides erroneously enable formatter if unspecified

[dyc3]

Summary

Previously, overrides could erroneously enable the formatter if it was supposed to be disabled, specifically in configurations like this:

{
  "formatter": {
    "enabled": true,
    "lineWidth": 20
  },
  "javascript": {
    "formatter": {
      "enabled": false
    }
  },
  "overrides": [
    { "include": ["formatted.js"], "formatter": { "enabled": true } },
    {
      // this override doesn't have a formatter section, so it would assume the global value, ignoring the language config
      "include": ["dirty.js"],
      "linter": {
        "rules": {
          "performance": {
            "noBarrelFile": "off"
          }
        }
      }
    }
  ]
}

This PR fixes that behavior by not forcing the override settings to have a Some value no matter what. I briefly looked into whether or not this affects the linter too, and I think it does, but I think that fix is outside the scope of this PR. This area of the code is likely going to be significantly changed by work on #2228. It's somewhat unclear what the semantics of overrides should be (if 2 overrides match the same file, do they get merged? do we take the first matching override and ignore the rest? the last one? etc etc)

Fixes #2924

Test Plan

Added a test with snapshots

[ematipico]

I think the fix is correct. It feels weird, but unfortunately, it's a side effect of our weird defaults.

[ematipico]

I was too hasty. @dyc3 would you like to send a PR with a changelog line?

[dyc3]

Can do: #2969