Update .js to .ts in documentation (#5558)

While reading the (excellent) documentation here, I noticed a few stale
references to `.js` files that are now implemented in TypeScript.

* Update `*.js` to `*.ts` in Markdown documentation.

* Update `app.js` to `out/dist/app.js` in
`docs/SystemdSocketActivation.md`.
_I managed to follow these instructions and locally run Compiler
Explorer through socket activation after that change._

* Update `*.js` to `*.ts` in the GitHub labeller config
`.github/labeler.yml`.
_Also correct some misnamed files here. I have checked the new version
with `ls $(grep -E "^ - " .github/labeler.yml | sed "s/ - //g")`. If I
were more familiar here, I would have liked to add a similar check to
the automated tests._

- Sheepishly add myself to the contributors list, despite this being a
tiny find-and-replace change.

---------

Co-authored-by: Jeremy Rifkin <51220084+jeremy-rifkin@users.noreply.github.com>

CONTRIBUTORS.md

@@ -138,3 +138,4 @@ From oldest to newest contributor, we would like to thank:
 - [Marc Auberer](https://github.com/marcauberer)
 - [Seyed Ali Ghasemi](https://github.com/gha3mi)
 - [Guo Ci](https://github.com/guoci)
+- [Rupert Tombs](https://github.com/Rupt)