Add webstore deployment action

[ColdSauce]

Hiya,

We built a GH action that makes deploying to the various webstores easier. I created a GH action here that uses it. So after the build step, it'll run our thing that deploys the generated ZIP files (the location of which you need to specifiy in SUBMIT_KEYS) to Chrome and AMO stores.

It looks like y'all don't wanna auto-release, so I made it a manual process, instead of auto-release.

For this PR to actually pass and not error, y'all gotta create a SUBMIT_KEYS github repository secret.

This secret is a json, with the schema defined here.

Here's a sample key:

{
  "$schema": "https://raw.githubusercontent.com/plasmo-corp/bpp/main/keys.schema.json",
  "chrome": {
    "clientId": "123",
    "clientSecret": "456",
    "refreshToken": "789",
    "extId": "abcd"
  },
  "firefox": {
    "extId": "123",
    "sessionid": "abcd"
  }
}

You can find instructions on how to get those keys in the schema, or if you use vscode, the schema should provide hint/intelisense when hovering over the json properties. If you need any help, lmk!