manifest.json: add gecko id

[willnorris]

This is a requirement for Firefox add-ons using manifest v3. The ID can take the form of an email-like identifier or a UUID. A quick survey of my installed add-ons shows that most developers seem to use the UUID, so I just generated one locally.

Updates #4

[ChrisRegado]

This results in "Errors" in Chrome (but it's actually just a warning, if you click in to look):

Unrecognized manifest key 'browser_specific_settings'.

Do we expect to need other changes to manifest.json for Firefox? Would it make sense to create a separate manifest file per browser? Or if we'll be adding a build script to handle packaging it into an installable extension, maybe we could patch the manifest in that build script?

[willnorris]

huh, well that's really annoying (that you can't have a manifest.json that is valid in both browsers). As best as I can tell, this is the only changes needed for Firefox. and at least initially, I'm building the extensions using npx web-ext build, so it's pretty minimal. I suppose we'll want a package.json to pin to a specific version of the web-ext module.

Let's hold off on this change for the moment then, and I'll see how much more is needed to package it for Firefox, and see if there is a good place to make this manifest.json change.