Add AppVeyor Windows-based CI (Continuous Integration) Service

[netweb]

AppVeyor is a Windows-based CI (Continuous Integration) Service ​https://www.appveyor.com/

With issues arising such as #44262 (Build tools: "grunt watch" throws fatal error on Windows)

By adding AppVeyor we can hopefully prevent some basic issues when running WordPress' build tools on Windows-based platforms.

In future iterations, we can investigate further build tools testing, be it Grunt tasks, Webpack, PHPCS, or PHPUnit

---

Once the forthcoming patch has been committed the following is required to set up the GitHub and Slack integrations for the ​https://github.com/WordPress/wordpress-develop repo :

• Slack - ​https://www.appveyor.com/docs/notifications/#slack
• GitHub - ​https://www.appveyor.com/docs/team-setup/#github-integration

---

Here is the latest CI build from my mirror: ​https://ci.appveyor.com/project/ntwb/wordpress/build/1.0.3

[pento]

In 44594:

Build Tools: Add AppVeyor integration.

AppVeyor allows us to test in a Windows environment. This first iteration tests that building works correctly.

Props netweb.
Fixes #44276.

[pento]

In 44595:

Build Tools: Tweak AppVeyor settings.

• Set grunt build as the build script
• Disable testing and deployment options
• Customise the Slack integration message
• Only show the Slack message when the status changes

See #44276.

[pento]

In 44599:

Build Tools: Don't send AppVeyor notifications on Slack on repeated successful builds.

See #44276.