Add support for .slnx files to Radzen Blazor Studio

[AdmiralSnyder]

I want to try radzen blazor studio with our 180+ projects solution to figure out if we can and want to use it, but we're using a .slnx file instead of a .sln file (this is a rather new VS feature, introduced in spring or summer 2025) and requires an additional nuget package (something something solution persistence) from applications wanting to read those files.

Describe the solution you'd like
just opening the .slnx would be awesome

Describe alternatives you've considered
I saved our .slnx as a .sln, and tried to open it, but got this error:

(no idea why you don't allow me to copy-paste the exception, it would have made things easier...) -> this should probably go into its own issue?

[AdmiralSnyder]

ah, forgot to put the version info there (another thing i can't copy-paste explicitly, one needs to know that ctrl-c works here - or try it...)

Radzen Blazor Studio 1.55.0