Description

Add the BuildStream configuration language.

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?type=code&q=NOT+is%3Afork+path%3A*.bst+kind+sources
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/apache/buildstream/blob/79ac919b5514db1fe750ef2fbc624e556570bc39/doc/examples/developing/elements/hello.bst
    • Sample license(s): Apache 2.0
  • I have included a syntax highlighting grammar: [URL to grammar repo]
  • I have added a color
    • Hex value: #006bff
    • Rationale: Taken from the official logo https://buildstream.build/
  • I have updated the heuristics to distinguish my language from others using the same extension.