Add new reporter lifecycle method to trigger after coverage report

[hi-ogawa]

Clear and concise description of the problem

As brought up in #9626 (comment), builtin HTML reporter and ui mode currently rely on special onFinishedReportCoverage method to wait for coverage html generation because currently available onTestRunEnd is too early.

vitest/packages/vitest/src/node/core.ts

Lines 1380 to 1389 in 86fad4b

	await this.coverageProvider.reportCoverage(coverage, { allTestsRun })
	// notify builtin ui and html reporter after coverage html is generated
	for (const reporter of this.reporters) {
	if (
	'onFinishedReportCoverage' in reporter
	&& typeof reporter.onFinishedReportCoverage === 'function'
	) {
	await reporter.onFinishedReportCoverage()
	}
	}

Suggested solution

Add Reporter.onCoverageReport method which gets triggered after coverage report.

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.