Suggestion: Add option to make generated files read-only

[GordonSmith]

On several occasions I have accidentally modified either the generated JS file, or the generated ".d." definition file.

Typically this happens inside vscode during a debug session, or within a chrome debug session. But it has also happened during regular edit/compile cycle (with the .d. file specifically).

[DanielRosenwasser]

Hey @GordonSmith, while I've definitely done the same, we've avoided getting involved with file permissions. For a similar request, see #14726 where we've suggested for users to incorporate a post-build step.