feat: introduce offset

[zhiyuanzmj]

close #296

Sets the offset property to adjust the incoming position for the following APIs: slice, update, overwrite, appendLeft, prependLeft, appendRight, prependRight, move, reset, and remove.

const s = new MagicString('hello world', { offset: 6 })
s.slice() === 'world'

const s1 = new MagicString('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