Upgrade CodeMirror to 5.62 and fix selections on code literals.

Review Request #11730 — Created July 17, 2021 and submitted — Latest diff uploaded


Review Board


This upgrades our version of CodeMirror from 5.48.2 to 5.62. There's a
number of fixes and additions for syntax highlighting, some edge cases
fixes for browser compatibility, and several other improvements.

We're now including the mark-selection addon, which we're using to fix
the styling of code literals (which would previously hide the
selection). The selected portion will now use the default selection

Tested standard CodeMirror behavior in the comment dialog and review
request fields. Didn't note any regressions.

Went through the release notes for CodeMirror to check what could
regress us, but didn't see anything that was a concern.

Tested selecting text and ensuring that code literals would properly
appear selected.