upgrade manifest-v3 for chrome

[daisuke8000]

Summary

Issue180

1. Update Manifest File

   • Update manifest.json to follow the MV3 format.
   • Adjust necessary APIs and permissions to be compatible with MV3.

2. Address MV3-Specific Changes

   • Adapt to the use of service workers and address API removals or deprecations introduced in MV3.
   • Add public/external-script.js to replace inline scripts, ensuring compliance with Content Security Policy (CSP).

Problem description

1. Warning Message
   The Chrome Web Store displays a warning indicating that the extension does not align with the "best practices for Chrome extensions" because it is still using Manifest Version 2.

   • There is a risk that this extension may soon cease to be supported by Google.

2. MV2 Deprecation Timeline
   Google has officially announced the deprecation of Manifest Version 2, and starting in 2024, support for extensions using MV2 will be discontinued.

   • Details can be found in the official documentation.

3. CSP Restrictions
   Due to restrictions imposed by Content Security Policy (CSP), the execution of inline scripts is blocked.

   • In response, inline scripts were extracted into an external file, external-script.js, that is now referenced externally.

4. Impact on the Project
   If this extension is not updated to Manifest Version 3, it may cease to function properly in the near future.

Pros/cons of approach implemented

[Pros]

• Improved Readability: More meaningful constant names and cleaner structure make the code easier to understand.
• Reduced Duplication: Common logic, like window.postMessage, is extracted into reusable functions, following the DRY principle.
• CSP Compliance: By extracting inline scripts to an external file, the implementation adheres to CSP restrictions, reducing the risk of security issues.

[Cons]

• Increased Complexity: Introducing new utility functions and abstractions might make the code harder to understand for first-time readers, as it adds more layers of indirection.
• Maintenance Overhead: Managing separate external script files may add an additional maintenance burden.

Checklist

• [x] Is this PR a reasonable size?

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