Fix multi-digit lambda args

[daveliepmann]

Fixes #671 by syntax-highlighting multi-digit lambda arguments, e.g. #(+ %10 %19).

Does not attempt to distinguish between %20 (which is both reader-legal and works) and %21 (which is reader-legal but does not compile) because that's complex and depends on Clojure compiler internals.

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[bbatsov]

Looks good to me. I guess I never realized people used double-digit lambda args. 😆

[daveliepmann]

I guess I never realized people used double-digit lambda args.

I never have and in 99.9% of situations would advise against them — I just noticed the regex didn't handle legal Clojure while fixing the keyword regex next to it 🤣