pw_presubmit: Handle banned words with underscores

Handle underscores around banned words. Before this CL "${BANNED}" was
blocked but "_${BANNED}_" was not.

Change-Id: Ieb4f6578036930a33669cf11007a245b7c89b751
Bug: 386
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/46342
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>