Expand documentation of Typescript testing

[haltman-at]

Issue

We seem to support writing Typescript tests in some form, but the details are unclear and it seems to be barely documented. There's a section for it on the page on writing Javascript tests, but it says basically nothing -- the "see for more" link in it just links back to the top of the same page!

Also, similar to @gnidan's idea in #3770 that there should be a "supported languages" page, maybe there should similarly be a "supported testing languages" page?

[gnidan]

We should also consider expanding our support, possibly by running ts-node when detecting TS tests

[haltman-at]

So, to make @gnidan's comment more explicit: Apparently currently you can't just write tests in Typescript, but rather it requires additional setup, which will need to be documented. (But it would also be nice if it didn't require that additional setup.)