gitattributes: ignore *.t and *.asl files

this will correctly mark the repository as an OCaml project (instead of
Raku or ASL which dominate by line counts).

see: https://github.com/github-linguist/linguist/blob/master/docs/overrides.md

.gitattributes

@@ -1,2 +1,4 @@
 *.asl   text eol=lf
 *.prj   text eol=lf
+*.t     linguist-vendored
+*.asl   linguist-vendored