Upgraded to manifest v3.

[sun]

Problem

• The ColumnCopy extension cannot be installed in Google Chrome anymore.

Proposed solution

1. Upgrade the extension to manifest v3.

Notes

• This PR gets the extension working again (tested in Google Chrome).
• Only minimum required changes, no code cleanups or other modifications.
• Given that Clipboard.writeText() is widely available now, I simplified the code to just use it instead of the clipboardBridge in an offscreen document.

Known issues

• Reloading the extension triggers the warnings (but seemingly not affecting the functionality) — fix suggestions welcome (might be covered by the tutorial linked below):

  Unchecked runtime.lastError: Cannot create item with duplicate id copyColumn
  Unchecked runtime.lastError: Cannot create item with duplicate id copyTable
  

Resources

• https://github.com/GoogleChromeLabs/extension-manifest-converter
• https://developer.chrome.com/docs/extensions/develop/migrate
• https://developer.chrome.com/docs/extensions/develop/migrate/api-calls#replace-unsupported-apis
• https://developer.chrome.com/docs/extensions/develop/migrate/to-service-workers#move-dom-and-window (clipboard)
• https://developer.chrome.com/docs/extensions/develop/concepts/messaging
• https://developer.chrome.com/docs/extensions/reference/api/offscreen
• https://developer.chrome.com/docs/extensions/reference/manifest/content-security-policy
• https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/content_security_policy

Tutorials

• https://www.coditude.com/insights/chrome-extension-manifest-v3-a-migration-guide/
• https://github.com/GoogleChrome/chrome-extensions-samples/blob/main/functional-samples/cookbook.offscreen-clipboard-write/background.js
• https://github.com/GoogleChrome/chrome-extensions-samples/blob/main/api-samples/contextMenus/basic/sample.js
• https://stackoverflow.com/a/79530265/811306

Fixes #35