Add support for an exclusion list similar to .gitignore

[raddog58]

Is your feature request related to a problem? Please describe.
I have two scenarios where this would help. Note that I currently only have this extension installed for VS 2013, but in the following first scenario, I suspect I'll hit the same thing in VS 2019.

So the first scenario is a DLL solution/project I maintain as a .NET Framework build in VS 2013 and as a .NET Core build in VS 2019. The .SLN for the VS 2019 .NET Core build shows up as the "File on disk is not included in the project" error when I build. There might be a way (not sure) to include but not build this in VS 2013, and when I install the extension in 2019 to do the same. However, I really don't want to mix the two projects preferring to add them as an exclusion/ignore entry.

The second scenario I ran into yesterday. I have a different solution with eight projects three of which are submodules I didn't create. When you right-click on the project and choose "Open Folder in File Explorer" one of the submodules' base folder contains the .git file which has the hidden attribute set. Not sure why it was created this way, but as it is I get the "File on disk is not included in project" error. I don't want to add .git to the solution, and I don't want to modify the submodule. For now I'm living with the error, but it would be helpful to set .git as an excluded file.

Describe the solution you'd like
I'd like a way to mark a file(s) as excluded from the not-included-in-project checking.

Describe alternatives you've considered
In scenario one I've considered adding the file that shows up as an error. In scenario two I'm living with the error for now.

Additional context
Add any other context or screenshots about the feature request here.

[raddog58]

Incidentally, double-clicking and/or right-clicking isn't bringing up the context menu with the "Exclude missing file from project" - I've included a screen shot.