WIP: add BuildStream to languages.yml

What does this MR do?

This adds BuildStream as a recognisable language so it can be detected in gitlab repos that use it

Unfortunately not too sure how to test it sorry, could someone point me in the right direction

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Shouldn't be any risk with this patch
• Checking that projects using BuildStream can be detected
• Should not require cross browser testing