[Built-in analyzer] String comparison is not properly quoted

### Background

This issue tracks one of the BuildCheck analyzers we would like to ship in-box with MSBuild.

### Goal

Implement an analyzer with the following rule: String comparison in conditions should use single quotes around the strings, e.g. `'$(MyProp)' == ''`.

### Notes

While not strictly needed for non-empty strings, the use of quotes makes the condition work for empty strings as well and is considered a good practice.