builtin: fix string.trim() (fix #13021)

[yuyi98]

This PR fix string.trim() (fix #13021).

• Fix string.trim().
• Add test.

fn main(){
	s := 'あいうえお'
	println(s)
	ts := s.trim('あい')
	println(ts)
}

PS D:\Test\v\tt1> v run .
あいうえお
うえお

[spytheman]

Excellent work.