Get script injection working for Chrome MV3 builds of the extension

[kzar]

Get script injection working for Chrome MV3 builds of the extension

Chrome Manifest V3 introduces the new scripting API[1] that can inject
scripts directly into the "main world" of a website. Along with that,
Manifest V3 prevents the old pattern of injecting a script element
into the DOM. Adjust to those changes here, to get our injected
protections working again.

Note: about:blank frame script injection is not yet working[2].

1 - https://developer.chrome.com/docs/extensions/reference/scripting
2 - https://crbug.com/1360392

Reviewer: @jonathanKingston

Steps to test this PR:

1. Navigate to the storage blocking test page.
2. Click "Store data", then "Retrieve data", expand the results, and note which are blocked.
3. Build MV3 extension: npm install && npm run dev-chrome-mv3
4. Install that in Chrome.
5. Load the test page again and retry the test in step 2.
6. Ensure that more third party cookies are blocked.

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications