feat(browser): allow custom HTML path, respect plugins `transformIndexHtml`

[sheremet-va]

Description

Closes #6394

This PR adds an option to specify your own HTML file for the Browser Mode. Now Vitest also always calls transformIndexHtml plugin hook when serving the tester file.

export default defineConfig({
  test: {
    browser: {
      testerHtmlPath: './component-testing.html',
    },
  },
})

Since Browser Mode is experimental, this PR can be merged with fixes and doesn't need to follow semver.

TODO

• [x] Tests
  • Non-existing file path
  • Error during transformIndexHtml
  • Valid HTML
  • Valid custom transformIndexHtml hook

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