feat: msix auto-updater

[bitdisaster]

Description of Change

This PR implements MSIX auto-updating according to electron/rfcs#21 . The new upater module supports auto-updating via:

• Mac.Squirrel JSON server format

{
  "url": "https://mycompany.example.com/myapp/package.msix",
  "name": "My Release Name",
  "notes": "Theses are some release notes innit",
  "pub_date": "2013-09-18T12:29:53+01:00"
}

• Mac.Squirrel static update file format

{
  "currentRelease": "2.0.0",
  "releases": [
    {
      "version": "1.0.0",
      "updateTo": {
        "version": "1.0.0",
        "pub_date": "2013-09-18T12:29:53+01:00",
        "notes": "Theses are some release notes innit",
        "name": "1.2.1",
        "url": "https://mycompany.example.com/myapp/package_1.msix""
      }
    },
    {
      "version": 2.0.0",
      "updateTo": {
        "version": "2.0.0",
        "pub_date": "2014-09-18T12:29:53+01:00",
        "notes": "Theses are some more release notes innit",
        "name": "1.2.3",
        "url": "https://mycompany.example.com/myapp/package_2.msix"
      }
    }
  ]
}

• direct MSIX url link
  https://mycompany.example.com/myapp/package.msix

Additonally, MSIX supports downgrading by setting allowAnyVersion. False by default.

 autoUpdater.setFeedURL({ url: 'https://mycompany.example.com/myapp/package.msix', allowAnyVersion: true }

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant API documentation, tutorials, and examples are updated and follow the documentation style guide
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Added support for MSIX auto-updating

[samuelmaddock]

I feel like this belongs elsewhere as a public API. I would be fine with it under app, but I'm curious what others think.

If we want to decide this later in favor of getting this out sooner, we could make it private by prefixing it as _getPackageInfo while not publicly documenting it.

[bitdisaster]

Yeah I wasn't sure either. My reasoning was that it gets info about the package, not the app. It's also not a hard requirement for this PR but might be useful for devs.

[samuelmaddock]

Might be useful to clarify this until we have a better named property

Suggested change

	if (process.windowsStore) {
	// windowsStore indicates whether the app is running as a packaged app, even outside of the store
	if (process.windowsStore) {

[samuelmaddock]

I could see this info being useful enough that I could be in favor of including it without the flag. As in, only requiring ELECTRON_ENABLE_LOGGING.

[bitdisaster]

Yeah I'm not sure about this either. I followed the model we have in the windows notification code.

[samuelmaddock]

Where does HelloMSIX come from?

[bitdisaster]

I added two lightweight MSIX packages in the test fixtures