rc detection modeline: optimize modeline pre-filtering

modeline-parse leads by matching an expensive regex against the entire buffer,
which can take a long time on huge files.

Perl takes too long on this regex and it seems not even ripgrep
optimizes the \z component

	$ ruby -e '10000.times { puts "a" * 10000 }' > big
	$ time rg --multiline --only-matching '\A(.+\n){1,5}|(.+\n){1,5}\z' big | wc -l
	10
	__________________________
	Executed in  419.81 millis
	   usr time  399.84 millis
	   sys time   20.78 millis

where

	$ time kak big -e q
	__________________________
	Executed in  179.19 millis
	   usr time  133.61 millis
	   sys time   53.50 millis

Let's lose the regex.

Fixes #4911

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