Add support for Elvish

[xiaq]

Usage in the wild:
https://github.com/search?q=extension%3Aelv&type=Code&ref=advsearch&l=&l=. There
are a couple of false positives, but the vast majority of the results are Elvish
scripts.

Code sample is taken from Elvish's standard library:
https://github.com/elves/elvish/blob/master/pkg/mods/epm/epm.elv, license BSD
2-clause.

Description

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on GitHub.com.
    • Search results for each extension:
      • https://github.com/search?q=extension%3Aelv&type=Code&ref=advsearch&l=&l=
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/elves/elvish/blob/master/pkg/mods/epm/epm.elv
    • Sample license(s): BSD 2-clause
  • I have included a syntax highlighting grammar: https://github.com/elves/elvish
  • I have updated the heuristics to distinguish my language from others using the same extension.

[xiaq]

@lildude Hi! Thanks for taking a look, I see you added the "Pending Popularity" which I assume means that the new language may not meet the popularity threshold.

Since the documented criterion is at least 200 unique repos, I wrote a program to test whether Elvish clears that criterion. The result is 224 repos as of today.

There are some false positives - not all .elv files are Elvish scripts. Clicking these repos randomly, roughly 1 or 2 out of every 10 repos are false positives. So the actual number of repos may be just a bit lower than 200.

Do you think this is close enough to the threshold, or should I try again the number of unique repos is clearly larger than 200?

[lildude]

We're temporarily using the requirements detailed in #5756 because GitHub's search is struggling with extension searches which often results in inaccurate, zero or duplicate results.

[xiaq]

I see, thanks. The number of results for .elv currently stands at 1794, hopefully it will be over 2000 soon :)

Maybe it's valuable to link to that issue from CONTRIBUTING.md - I opened #5941 for this.