MagicString.indent is slow

[Victorystick]

According to rollup/rollup#723 (comment), MagicString.indent can take up quite a bit of Rollup's bundle time. It may be worthwhile to try to optimize it.

[Rich-Harris]

Yeah. It's not any slower than any other operation, it's just that it needs to do a lot more of it (Rollup only needs to make small edits here and there until indent(), at which point every line is affected) – would definitely be good to make it faster, but it may be the case that we're better off just disabling indentation by default. Won't be able to take a look any time soon (am on a harsh deadline hence my recent radio silence!) but keen to improve this.