Upgrade CodeMirror to 5.13
Upgrade CodeMirror to 5.13

At least version 5.11 is needed to enable JSX mode, but we
couldn't use 5.11 or 5.12 due to a known issue with
bidirectional text [1]. Release 5.13 included Michael Zhou's
fix for [1].

Now that 5.13 is released and deployed to Maven, upgrade to that
version. JSX support, along with many more language modes that
we missed during previous upgrades, will be added in a follow-up
change.

[1] https://github.com/codemirror/CodeMirror/issues/3846

Change-Id: I37314d9d4f8aef98fe724a999c08f09aa7d411fa

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