Test addon should install coverage provider out of the box

[shilman]

Currently when you install the test addon, it does not install a coverage provider. When you try to run coverage it fails. To debug you need to look at the console, which is non-intuitive. Then you need to add a coverage provider (it suggests V8) and then restart.

We should just install the coverage provider out of the box. And if we think istanbul is the better choice, we should install that and configure Vitest accordingly. (I'm still on the fence about v8 vs instanbul!).

[ndelangen]

If we do this, than there's no point in this PR: #29763 right?

@shilman What's the decision here?

• Do we try to setup with istanbul and only install that?
• Do we install both into the user's project?
• Do we auto-install v8, because that's the default?

I have to assume there's a reason why the vitest team decided to make both providers are opt-in to install.

[JReinhold]

Decision:

• During the addon's postinstall, detect if either @vitest/coverage-v8 or @vitest/coverage-istanbul is installed. If neither of those are, automatically install @vitest/coverage-v8, with a log line explaining why.
• ... That means we won't try to be smart and detect which provider is potentially configured already, we assume that if a package is installed, it is correctly configured.
• This can still fail in a few edgy scenarios, which is why we still wan't to move forward with Addon Test: Clarify message when vitest detects missing deps #29763 to ensure a good error message at runtime.