Improve AppVeyor Configuration

[manuth]

Adding support for WSL to AppVeyor caused the appveyor.yml file to look quite complicated.
If applied, this PR will simplify the appveyor.yml-file by temporarily adding a Cmdlet for invoking commands either using . (source) or wsl depending on the current configuration.

This allows us to use the same commands for both WSL and Native, leaving only install-commands as separate sections.

Further Explanation

The GetCI.psm1-file contains a PowerShell function called Get-CICommand. This takes a string containing a command (such as npm install, sudo npm install -g rimraf) and converts it for the appropriate platform:

• For WSL, a wsl is prepended
  Example: sudo npm i -g rimraf => wsl sudo npm i -g rimraf
• For Windows, if the first argument is sudo, it is removed as the sudo command doesn't exist on windows and all commands on AppVeyor are executed as admin anyways.
  Example: sudo npm i -g rimraf => npm i -g rimraf

Additionally the ci.cmd script is added to the PATH-variable.
The ci command accepts arguments representing a command (such as npm install, git reset etc.), converts it for the appropriate platform using Get-CICommand (mentioned before) and executes the resulting command using cmd /c (a command similar to bash -c)

Related to #1786

[coveralls]

Coverage remained the same at 97.739% when pulling e1ed323 on manuth:wsl-fix into 9dfef28 on benmosher:master.

[manuth]

I'm almost done, only some command-line outputs during FOR commands are bugging me... hope my next commit will fix this.

[manuth]

@ljharb alright, I'm done 😄
Could you please force push this PR back to 0d2fa00?
My most recent commit didn't change anything, so it's not necessary.

[manuth]

@ljharb alright, I'm done now 😄
Is it actually allowed to create a PR for adding VSCode workspace files (such as debug config and stuff)?

[ljharb]

@manuth no; files related to your personal IDE should go in your personal global gitconfig, instead of adding them to every repo you touch :-)

[ljharb]

Thanks!