Possible race condition when initializing CodeMirror leading to double initialization

Right now nothing prevents calling twice the initilization of SyntaxHiglighting (and so CodeMirror) on the same textarea.
It's more likely to happen in XWiki in recent changes performed related to the toolbar of the wiki editor driven by events, and so it led to the regression XWIKI-23722.
We can fix that easily by flagging a textarea being augmented and preventing to augment it again if it is.