regex: fix regex.split() (fix #16876)

[yuyi98]

This PR fix regex.split() (fix #16876).

• Fix regex.split().
• Add test.

import regex

fn main() {
	mut r := regex.regex_opt('-') or { panic(err) }
	println(r.split('a-'))
	println('a-'.split('-'))
	println(r.split('-a'))
	println('-a'.split('-'))
}

PS D:\Test\v\tt1> v run .
['a', '']
['a', '']
['', 'a']
['', 'a']

[penguindark]

Good work! For me it is ok!

[penguindark]

Good for me, thanks for the fix.