Allow trailing commas in solution filters

[Youssef1313]

Have a slnf with trailing comma and try to restore, you get the following error:

Solution file error MSB5025: Json in solution filter file "path\to\filter.slnf" is incorrectly formatted.

See dotnet/aspnetcore#31438

[benvillalobos]

Where does an issue like this originate? Where did the comma come from?

[Youssef1313]

@benvillalobos For aspnetcore repository, it should have been added manually.

[rainersigwald]

We should match the VS behavior here: if it allows trailing commas so should MSBuild.

cc @Forgind