fix: C8 sourcemaps

[Monkatraz]

Honestly, I'm not exactly sure what black magic is happening with C8 and the like, but as far as I understand this is a needed change. I don't think C8 was actually doing any source mapping prior to this, because it cached the coverage map when report.getCoverageMapFromAllCoverageFiles() was called.

Here is what Vitest is generating for source maps:
sourcemapthingy.zip

use this site to view it: https://sokra.github.io/source-map-visualization

You'll notice that the source mapping seems pretty much perfect, at least to me. I think any further issues with coverage are related to C8 itself.

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[Monkatraz]

I think this fixes the same issue as #342

EDIT: I actually think both this and #342 are required because the source mapping isn't otherwise available in the modules (which borks breakpoints)

[Monkatraz]

Further investigation:
This esbuild issue I think is relevant to some of the weird coverage issues.

If we take a look at this image:

You can see how the source is mapped pretty weirdly to the generated file.

That results in weird coverage that looks like this:

Really, the issue is two fold:

1. The generated source mapping is not very dense
2. Some of the mappings are bizarre and probably confuse the reporter

I see a couple "solutions" for this. One is to use tsc rather than esbuild when running with coverage, another is to use Istanbul directly. I don't like the latter, nor do I like the former, but out of the two, I think using tsc is probably better? If people are running with esbuild plugins I could see that causing issues though.

Another reason to use tsc is that esbuild actually rips any comment that isn't a legal comment, which means you can't actually use the /* c8 blah */ comments unless I'm missing something.

[Monkatraz]

Okay, yeah, tsc emits a waaay more precise source map:

However, it breaks if I run it through the ssr mode. I presume that's because of the hacky tsc plugin I'm using.