Issue #18: Add PSScriptAnalyzer linting CI on every pull request

[jantari]

This doesn't do unit tests, just linting / static analysis with PSScriptAnalyzer so it doesn't completely fix #18

I've made a few things configurable through the env-variables.

This runs on every PR and leaves a comment with the analyzer results.

An example of what the comment is formatted to look like is here: https://github.com/jantari/PowerShell/pull/1