Addon Vitest: Use `enableCoverage` API instead of re-creating Vitest instance

[AriPerkkio]

Follow-up from #36200 (review).

Storybook 11 will drop Vitest 3 support. That allows Storybook to utilize enableCoverage API from Vitest 4. This will simplify Storybook's code coverage toggle functionality a lot, and speed up end-user experience.

storybook/code/addons/vitest/src/node/vitest-manager.ts

Lines 384 to 385 in eb9f3f8

	} else if (currentCoverage !== coverageShouldBeEnabled) {
	await this.restartVitest({ coverage: coverageShouldBeEnabled });