Support VSCode source.fixAll.

[Rapougnac]

Describe the bug
In a project with a .vscode/settings.json file:

{
  "editor.codeActionsOnSave": {
    "source.fixAll": "explicit",
  }
}

This will not run custom_lint quick fixes on save, but successfully runs quick fixes of the base analyzer

To Reproduce
Any custom lint that has a quick fix.

Expected behavior
The most important quick fix should run

[rrousselGit]

This isn't something that custom_lint can deal with.
The issue is analyzer_plugin ; which doesn't offer a way to react to "fix all" events.

[rrousselGit]

dart-lang/sdk#55887