Also add multiple selection with alt key
Also add multiple selection with alt key

CodeMirror 6 by default uses the ctrl key to allow for multiple
selection. DevTools historically allowed the alt key to do the same.
Therefore, with the instructions from
https://github.com/codemirror/codemirror.next/issues/765#issuecomment-1062010543
update the editor configuration to allow both ctrl and alt keys to be
used.

R=1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org

Fixed: 1304070
Change-Id: I2c9fb03dca632819805a66c4257fd460b7e0196f
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3513712
Commit-Queue: Tim Van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Auto-Submit: Tim Van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Reviewed-by: Yang Guo <1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org>
Commit-Queue: Yang Guo <1cf0af8245a3a17a632a2c73531392b0f9761c76@chromium.org>

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