Add 'gel' extension and alias to EdgeQL

[1st1]

Description

EdgeDB got recently renamed to Gel [1]. The schema language extension changed from .esdl to .gel. This PR adds .gel as a recognized file extension and gel as a recognized code-block language selector.

[1] https://www.geldata.com/blog/edgedb-is-now-gel-and-postgres-is-the-future

Checklist:

• [x] I am adding a new extension to a language.
  • [ ] The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.gel+%2Fmodule+%5Cw%2B+%5C%7B%2F
      • This is a very recent change in the defaults regarding EdgeDB being renamed to Gel, so realistically we expect similar adoption of this format to what we have for older extension .esdl (https://github.com/search?type=code&q=NOT+is%3Afork+path%3A*.esdl). The new extension (taking heuristics into account) is currently ~95 hits in the search, though.
  • [x] I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • Self-written
    • Sample license(s):
      • MIT
  • [x] I have included a change to the heuristics to distinguish my language from others using the same extension.

[1st1]

@lildude, sincere apologies for not following the process. As an open source maintainer myself I should have had better judgement.

We've updated the commit / PR message, please take another look.

[vpetrovykh]

I've added a more representative example. It's still synthetic, but it's based much more closely on actual real schema files.