ChromeVox compatibility

[aleventhal]

Since we can't use the native messaging messaging api to communicate with ChromeVox, we should just open an ordinary port.

Ideally, ChromeVox would be able to bundle js2at functionality without requiring the additional extension. However, this would require js2at to be more mature and ready to be bundled with Chrome.

So for now, let's just open up the additional port.

[aleventhal]

Send to Chromevox port, id is mndnfokpggljbaajbnioimlmbfngpief

This is listed in Chrome source in contents.cc here:
https://cs.chromium.org/chromium/src/extensions/common/constants.cc?sq=package:chromium&g=0&l=100

[aleventhal]

Landed code that connects to ChromeVox via a port.
ChromeVox can listen with the following code:

chrome.runtime.onConnectExternal((port) => {
    if (port.name !== 'js2at->cvox')
      return;  // Not handled.

Also, ChromeVox will need to white list js2at under externally_connectable in the manifest, like:

externally_connectable:  {
   "ids": [
      "jpgoldinadnmhfknenolkgbnockemnid"
  ]
}