QuickBASIC was recently added as a language supported on GitHub for files with the .bas extension. It doesn't include .bi files since there isn't enough usage on GitHub at the moment to justify the inclusion. However, by adding the following lines to your .gitattributes file, it will mark .bi files as QuickBASIC.

*.bas linguist-language=qb64
*.bi linguist-language=qb64

Note that I've also added a line to override the dectection for .bas file in case the automatic detection isn't preforming correctly for certain files.