[Built-in analyzer] Property that starts with an underscore is set

### 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: Properties whose name starts with an underscore should not be set in project files that are part of the project (as opposed to SDK project files).

### Notes

By convention, the leading underscore is used to mark properties as internal to common targets or SDK, and not expected to be modified by the user.