add trim method inference @ String.

[igalklebanov]

Hey 👋

This PR adds return type inference for String's .trim/.trimStart/.trimLeft/.trimEnd/.trimRight.

const moshe = '\t\n\r moshe \r\n\t'

moshe.trim() // 'moshe'
moshe.trimStart() // 'moshe \r\n\t'
moshe.trimLeft() // 'moshe \r\n\t'
moshe.trimRight() // '\t\n\r moshe'
moshe.trimEnd() // '\t\n\r moshe'

const notMoshe: string = moshe

notMoshe.trim() // string
notMoshe.trimStart() // string
notMoshe.trimLeft() // string
notMoshe.trimRight() // string
notMoshe.trimEnd() // string

[mattpocock]

Closing for reasons described here