V8 Coverage: Exclude empty lines, comments, TS types

[AriPerkkio]

Clear and concise description of the problem

There's now a PR for improved v8 coverage remapping in v8-to-istanbul:

• feat: exclude lines that are missing from source maps istanbuljs/v8-to-istanbul#244.

There are some coverage reports in the linked PR that show what kind of changes it would cause.

• Comments are excluded (esbuild doesn't add comments in source maps, tsc does though)
• Empty lines are excluded
• Typescript typings are excluded (except enums that compile to actual runtime code)
• HTML-like templates from transpiled languages like Svelte and Vue are excluded, as long as they don't contain code that those compilers consider (user's) runtime code.
• Ignore hints (/* v8 ignore ... */) exclude the lines instead of marking them as covered. Exactly as istanbul-lib-instrument does.

Related issues:

• Solves many issues listed in Coverage: give the choice between c8 and nyc #1252
• Relates to fix(coverage): v8 to ignore type-only files #5328, types are now also excluded from the files that were run

Suggested solution

Typically PRs in istanbuljs organization take long time to land so we could apply the changes as pnpm patch and bundle v8-to-istanbul into @vitest/coverage-v8 package.

At first we might want to introduce this as opt-in coverage option and enable it by default in V2. The changes that this introduces can affect line coverage in such ways that projects using coverage.thresholds may now fail in CI - exactly as they should do. Line coverage is now more accurate as comments don't increase it.

Alternative

No response

Additional context

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