chore: add a test coverage report

[sadjow]

pnpm test:coverage runs the Vitest suite with the V8 coverage provider and reports coverage for src/. It prints a per-file table and writes an HTML report to coverage/index.html, which Git, Prettier, and ESLint already ignore.

On main, the suite covers 89.68% of lines in src/:

File	Lines	Branches
utm.ts	92.85%	100%
parameters.ts	90.9%	60%
plugin.ts	90%	70.14%
options.ts	81.25%	58.33%
composables.ts	0%	100%

The numbers count only code that runs inside the Vitest process: the unit tests, and the module setup that runs while fixtures build. Code the browser integration tests exercise in the page or in the fixture's Nuxt server isn't counted, which is why composables.ts shows 0% even though the integration tests call useNuxtUTM(). CONTRIBUTING explains how to read the report.

@vitest/coverage-v8 must match the installed Vitest version exactly, so a new Dependabot group updates vitest and @vitest/* together instead of in separate PRs that would leave them mismatched. The command is opt-in for now. CI runs pnpm test as before, and no coverage threshold is enforced.

Validation: pnpm test:coverage (128 tests), pnpm format:check, and pnpm lint --max-warnings 0 with a generated coverage/ folder present pass.