improve handling of mime associations

[akosyakov]

What it does

• fix Recompute editors on mime associations change #5017: detect languages on mime associations change
• fix files.associations is breaking PHP features and probably affecting other languages too #7791: update mime associations on languages change

How to test

• open an editor, change file.associations check that editor reflect changes without reopening
• test against steps from files.associations is breaking PHP features and probably affecting other languages too #7791 (comment)

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[akosyakov]

@joazvsoares Could you try against this PR?

[vince-fugnitto]

I've verified the changes against #7791 and they work correctly 👍

---

I've performed the following steps:

1. added the necessary php builtin and external vscode extensions (as described in the issue).
2. created both a test.php and test.txt file in the workspace.
3. created a file association such as:

{
  "files.associations": {
      "*.php": "php",
      "*.txt": "php"
  }
}

4. verified that both test.php and test.txt are both syntax-highlighted and contains completion as if they were php files.
5. verified that the previous step remains true when the application is reloaded.

[joazvsoares]

Thanks for jumping into this and for getting the fix ready so fast!