Add SummaryReporter for vitest

[acelaya]

This PR adds a SummaryReporter for projects using vitest, which extends the default reporter but suppressing the output generated at the end of every file.

That output was a bit annoying for us, since we pre-bundle a single test file, causing a big chunk of output to be printed at the very end of the test execution.

Since not all projects are using vitest yet, this PR also defines vitest as an optional peer dependency, and exposes vitest-related symbols from a new @hypothesis/frontend-testing/vitest entry point.

[acelaya]

I tested these changes via yalc and seems to be working properly.

[robertknight]

Can you add a comment with an example of the output using the DefaultReporter versus this new reporter for a repository where we've enabled Vitest? Also I had a query about how the changes here relate to the BasicReporter.

[robertknight]

I see that Vitest also has a BasicReporter. How does the output from that differ from what we're doing here?

[acelaya]

The basic reporter is the same as the default reporter, but with the summary disabled. Basically the opposite of what we want.

[acelaya]

Sorry, my bad. The basic reporter seems to be the same we want, except that it does not print the progress in real time, only at the end.

basic-reporter-2025-04-24_10.54.48.mp4

That makes me think that the summary is not what I have been calling "summary", but the output from every file. Perhaps we should find a better name for our reporter?

[acelaya]

By the discussion here, it seems what we want is in fact the summary, so SummaryReporter sounds like a good name vitest-dev/vitest#7881

[acelaya]

Can you add a comment with an example of the output using the DefaultReporter versus this new reporter for a repository where we've enabled Vitest?

A comment where?