Add support for .slnx files in the action "Open Visual Studio Solutions"

[T-Emile]

This PR adds support for the new slnx file format for Visual Studio solutions.

In addition to adding *.slnx in RepositoryActions.json, this PR also:

• Adds support for combining multiple patterns separated by a ; to support both .sln and .slnx in one file association.
• Ignores entries located in a hidden folder (that start with a dot). This is to exclude the Project.slnx folder created by Visual Studio inside the .vs folder.