vim-patch:9.1.1957: filetype: bpftrace files are not recognized (#36838)

Problem:  filetype: bpftrace files are not recognized
Solution: Detect *.bt files as btftrace filetype,
          include a btftrace filetype plugin (Stanislaw Gruszka)

closes: vim/vim#18866

b60b33a9dc

Co-authored-by: Stanislaw Gruszka <stf_xl@wp.pl>

runtime/lua/vim/filetype.lua

@@ -267,6 +267,7 @@ local extension = {
   bl = 'blank',
   blp = 'blueprint',
   bp = 'bp',
+  bt = 'bpftrace',
   bs = 'brighterscript',
   brs = 'brightscript',
   bsd = 'bsdl',