Add `.bxl` to Starlark

[lf-]

https://buck2.build/docs/bxl/

Description

Buck2 has PACKAGE files which do directory specific configuration. I am not familiar enough with Bazel to draw a comparison to WORKSPACE, but I think the Buck2 equivalent is a combo of buckconfig and PACKAGE. BXL is the Buck eXtension language and it lets you write programs in Starlark which query your build graph.

Checklist:

• [x] I am adding a new extension to a language.
  • [x] 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*.bxl
  • [x] I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/facebook/buck2/blob/c779e6f75b80779a4951f99774040a324ff32f2e/examples/bxl_tutorial/part2.bxl
    • Sample license(s): Apache 2 & MIT
  • [x] I have included a change to the heuristics to distinguish my language from others using the same extension.

[lf-]

I have looked at the statistics for popularity. There seem to be no real false positives; some HTML temp files people mysteriously checked in, some seemingly randomized file names. I can't really determine whether there are hundreds of repositories with the UI, but there are hundreds of true positive matches for the search.