coverage: Switch to raw coverage files (#40013)

It turns out that there really aren't any really good generic coverage
reporting tools, the best you can get is something that highlights
covered lines without function/method stats, branch info, or anything
else fancy.

To provide useful coverage output, we're going to look instead at
generating separate coverage dashboards for PHP (PHPUnit) and JS
(jest/istanbul), each generated by those tools so we can get all the
useful info possible. To do that, we'll need all the coverage commands
to generate the tools' raw data formats rather than a lossy format like
clover.

As part of this conversion, we're adding `jest.config.cjs` everywhere so
we can consistently set `collectCoverageFrom`. By default the base
config will include non-test files under `src/`. Various packages need
additional dirs such as `components/`, and a root-level `index.jsx` or
`global.d.ts` seems common too. Then, since we have config files
everywhere, we can move `coverageDirectory` and `coverageReporters` into
the configs too.

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/11642430412

Upstream-Ref: Automattic/jetpack@161c7a6

composer.json

@@ -30,7 +30,7 @@
 		"post-install-cmd": "WorDBless\\Composer\\InstallDropin::copy",
 		"post-update-cmd": "WorDBless\\Composer\\InstallDropin::copy",
 		"test-coverage": [
-			"php -dpcov.directory=. ./vendor/bin/phpunit --coverage-clover \"$COVERAGE_DIR/php/clover.xml\""
+			"php -dpcov.directory=. ./vendor/bin/phpunit --coverage-php \"$COVERAGE_DIR/php.cov\""
 		],
 		"test-php": [
 			"@composer phpunit"