Add support for Magik and PKl languages that are not handled by Linguist

[mmanela]

There are languages which Linguist doesn't plan to handle but we want to still support. This change allows us to define our own list of these languages as a fallback for when linguist (and go-enry) do not know about a language.

Part of https://linear.app/sourcegraph/issue/GRAPH-622/magik-search-filter-support

[keegancsmith]

Your approach looks correct to me. However, I believe we want to ensure both sourcegraph and zoekt use the same logic for file type identification. In that case it may be fine for us to just import zoekt, but more than likely @varungandhi-src has a better idea. I know he was exploring consistency in language identification a few months ago.

[jtibshirani]

However, I believe we want to ensure both sourcegraph and zoekt use the same logic for file type identification.

@mmanela is making corresponding changes in Sourcegraph too: https://github.com/sourcegraph/sourcegraph/pull/63110. I am +1 to this approach as the best near-term solution.

I don't think we should block this work on pulling out our own language detection library. Although I'd love if we had such a package in the future. And I'd also love for there to be consistency between scip-ctags and our language detection (maybe these could be bundled into the same package?)

[jtibshirani]

Last review comment from me: I'd love if we added a quick check in TestSearchTypeLanguage within index_test.go to exercise this code path!

Then it looks ready to merge -- do you have permissions to do that?

[varungandhi-src]

I think it's OK to have this logic here, since we can change this later easily. I'll see if we can move our language detection logic to a separate repo.