[Feature Request]: cppchecker plugin - handling of environment variables

[audrius-u]

Describe your idea in details

Currently cppchecker plugin configuration handles three variables, namely ${cppcheck}, ${WorkspacePath} and ${CurrentFileFullPath}. The first one is not handled properly - I have cppcheck installed on my system, set the OS environment variable cppcheck to point to actual location of cppcheck.exe, and set the same in CL's Environment Variables - despite this, CL failed to see cppcheck until I added its location to system PATH. However, I would prefer to have system's PATH less cluttered. Additionally, I want to run cppchecker with ${ProjectPath}/compile_commands.json set per project, not per workspace as I have multiple projects in a single workspace, and ${ProjectPath} is not resolved either. Please add an ability to handle environment variables, or at least ${ProjectPath} support if possible. Thank you!