Rework debug id injection

[loewenheim]

Injection is incredibly buggy, mostly because we only inject at a line boundary, even though sometimes the correct place is in the middle of a line. This creates frustration for users and hurts debug id adoption. Splitting lines apart would mean we would need to edit the sourcemap in a nontrivial way, however.

A possible way forward—my preferred option, if it works out the way I imagine it—is as follows.

1. Use the Rust port of magic-string to inject in exactly the right place and generate a sourcemap from the injected file to the pre-injected one. In my preliminary tests this works nicely.
2. Add a composition operation to rust-sourcemap. Given a sourcemap ab.map from file a to file b and a sourcemap bc.map from file b to files c₁, …, cₙ, this would produce a sourcemap ac.map from a to c₁, …, cₙ by composing the mappings from ab.map and bc.map.
3. Compose the sourcemap obtained in (1) with the one we already have to obtain a correct sourcemap from the injected file to the unminified files.

Tasks

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1. (by 2023-07-28) ref(injection): Rebuild fixup_js_file on top of magic-string and sourcemap composition #1693
2. (by 2023-08-04) feat: Implement sourcemap composition rust-sourcemap#67
3. (by 2023-08-11) Integrate the previous two items into a corerct injection procedure (see 1)
4. publish: getsentry/[email protected] publish#2575
   accepted +1