Add mannually install option.

[neo-sam]

avoid CRX_REQUIRED_PROOF_MISSING for vanilla Chromium browsers.

bandicam.2022-05-14.19-22-19-136.mp4

[NeverDecaf]

Looks good but I have a few concerns:

1. As it is, this will only work on the Microsoft Edge Store, is this intentional?
2. Opening chrome://extensions and showing a notification for every download needs to be limited because if a user uses the "Import" or "Update all extensions" features, they will be spammed with new tabs and notifications.
3. "Manual" implies the opposite, "Auto," and I'm afraid that will cause more confusion about this extension, I would prefer a more precise name for this option, for example: Always download CRX files.

For 1, to make this work on the Chrome Web Store, you will need to modify promptInstall to always use the non-webstore option if this feature is enabled. Then, modify the listener chrome.downloads.onChanged.addListener in background.js to use your new download method (or do nothing) if this feature is enabled. Finally, you would need to modify the Edge section of inject.js (line 152) to use promptInstall as well.

I can make the above changes myself if you prefer, but first I want to get a working ManifestV3 version of this extension, then I'll merge this PR.