Add Github Action to deploy to web stores

[ColdSauce]

Hey @revir, we created a Github action to make it easier to publish extensions to the various web stores.

Thought you might find it useful so I added a Github workflow that will build the extension, zip it up, and publish to the web stores on dispatch.

The only thing you would need to create is 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/v1/keys.schema.json",
  "chrome": {
    "clientId": "123",
    "clientSecret": "456",
    "refreshToken": "789",
    "extId": "abcd"
  },
  "firefox": {
    "apiKey": "123",
    "apiSecret": "abcd",
    "extId": "foobar"
  }
}

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 in setting up the keys, feel free to @ me.

Otherwise, if this doesn't seem necessary, feel free to close the PR!