chore(ci): run browserstack against v3 feature branches (#4082)

this commit updates our ci pipelines to run against feature branches of
the format 'v3.(VAL)-feature' where VAL matches zero or more characters
(not matching the '/' character)

this is being done so the team can try out feature branches in v3 of
stencil and take advantage of browserstack tests

Author: rwaskiewicz

.github/workflows/test-browserstack.yml

@@ -4,6 +4,7 @@ on:
   pull_request_target:
     branches:
       - 'main'
+      - 'v3.*-feature'
   push:
     branches:
       - 'main'