feat: introduce withOffset

[zhiyuanzmj]

close #296

Returns a proxy instance that modifies the incoming position by adding an offset.
Intercepted API: slice, update, overwrite, appendLeft, prependLeft, appendRight, prependRight, move, clone, snip, reset, remove.

import { MagicStringOffset } from 'magic-string'

const s = new MagicString('hello world').withOffset(6)
s.slice() === 'world'

const s1 = new MagicStringOffset('hello world', { offset: 6 })
s1.slice() === 'world'
s1.offset = 8
s1.slice() === 'rld'

[antfu]

This makes me wonder, if we should have the offset function directly in the MagicString class and make it immutable. But yeah it's also tricky as it's different from .clone this would write back to the original instance

[antfu]

I am also not sure about this usage, it feels like we should either make it always immutable (always call .withOffset to create a new instance) or always mutable (that brings us back to the original issue).

[antfu]

Yeah, after a second thought, I think the original design might be better. We shouldn't think too much about immutability here, as the MagicString itself is designed to be mutable. Let's check back to a single class with the mutable offset property then. Sorry for the back and forth

[zhiyuanzmj]

No matter, I will add an offset field in the afternoon.