Add .stylelintignore
Add .stylelintignore

This makes it easier to start auto-fixing the stylesheets as separate
CLs as we prepare to enable stylelint.

Bug: chromium:1083142
Change-Id: I9bc03f9f68e5c89e625bac0cc1b71684df2d6375
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2203452
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Commit-Queue: Mathias Bynens <17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org>

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