[Fix #581] Fix font locking for keywords starting with a number

[OknoLombarda]

Fixes #581

Context

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• The commits are consistent with our contribution guidelines.
• You've added tests (if possible) to cover your change(s). Bugfix, indentation, and font-lock tests are extremely important!
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• You've updated the changelog (if adding/changing user-visible functionality).
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Thanks!

[bbatsov]

Thanks! And sorry for forgetting to merge this earlier!