bug#68246: 30.0.50; Add non-TS mode as extra parent of TS modes

[Stefan Monnier]

>    (set-language-for-mode 'foo-ts-mode 'foo)

Maybe we want to introduce this concept, indeed.

maybe we want to that notion of "language" from elsewhere, such as
the one used in LSP?
Or maybe we want to take it from MIME types?
I'm sure there are other options out there.

Problem is: they come with their own complexities and corner cases.
After all, this is inevitable when you create a taxonomy.
IOW, while we *may* want to add support for an explicit notion of "file
type", it's a whole problem in itself and it will not solve all
our problems either.

In the mean time, I think `derived-mode-add-parents` is worth a try.
As mentioned in some message up-thread, I'm not 100% confident that it
won't introduce serious breakage.  But I think we do need more
experience and installing my patch is a good way to do that.


        Stefan