Fix diagnostics on save or on format

There are some diagnostics which are safe to fix everywhere (like field-shorthand or useless-braces) and there are some diagnostics which you may not mean to fix some times, but even then, you can recover the previous state easily (unused-imports are in this category IMO, you can easily import removed things later with auto-import assist).

This issue suggests adding a list of diagnostics, empty by default, which allows r-a to fix them on save (or on format, or on another action, or configurable between them) automatically. We may still not fix all instances of diagnostics (for example, if user puts auto-import in that list, we may not fix those with multiple candidates). I think adding this config helps in development speed.