Add multiplex addon
Add multiplex addon

Use case is codemirror breaks when using mutiple languages in one file

multiplexingMode is required by htmlembedded so needs multiplex addon.

Got this javascript error:

- "TypeError: undefined is not a function (near '...a.multiplexingMode...')"

This change fixes it.

See [1]

[1] https://github.com/codemirror/CodeMirror/blob/04d39f236a541104c3fccdfdf6ea164d6e4c74e7/mode/htmlembedded/htmlembedded.js#L18

Change-Id: I9c79d77c902dbb22271f33d9d6e2f9e50726d411

GerritCodeReview/plugins_codemirror-editor,GerritCodeReview/plugins_codemirror-editor,GerritCodeReview/plugins_codemirror-editor