Critical CSS Gen: Fix potential memory leak

[dilirity]

When this package is used in Playwright mode, there's a potential memory leak when playwright fails to open a tab but it opened one before it. This PR aims to fix this by introducing a cleanup for the playwright interface.

Proposed changes:

• Add cleanup method to playwright interface, to try and close pages;
• Fix not awaiting cleanup when css generation is done.

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• This is best tested with the local version of Boost Cloud;
• Make sure to run it using cloud dev --dev-packages to tell docker to use the version of this package within the monorepo, instead of the production one.
• Make sure critical css generation still works via the Jetpack Boost UI.

[github-actions]

Thank you for your PR!

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