gn-mode: Add matching for variables in strings.

This adds font-lock regexps to match the three different embedded
variable syntaxes. I added faces for these constructs, and reorganized
variables so they're now all part of a group for custom.

BUG=none

Review URL: https://codereview.chromium.org/1057993003

Cr-Original-Commit-Position: refs/heads/master@{#324074}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 3e301cc25cf6f62dfd3b7df36a62db87a7292926
1 file changed
tree: 4e64ea996753166021e4a7eb23859ea923987d8c
  1. tools/