Optimize context valid identifier check

[Richtermeister]

This might seem like a micro-optimization, but we are using this library in a tight loop with a large input map, and the regex usage is adding up. This PR contains a commit with the benchmark used.

goos: linux
goarch: amd64
pkg: github.com/flosch/pongo2/v6
cpu: Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
BenchmarkCheckForValidIdentifiers/regex-8         	 5310591	       231.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkCheckForValidIdentifiers/char_check-8    	127916318	         9.171 ns/op	       0 B/op	       0 allocs/op

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