Support slnx files to for dotnet-sln project types

[carlosdagos]

The extension slnx is an atlernative file type to sln to describe dotnet solution projects, see:

https://devblogs.microsoft.com/dotnet/introducing-slnx-support-dotnet-cli/

Support slnx projects in the same way as sln projects, by checking that a file exists with the relevant file extension.

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by checking the relevant checkboxes):

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (eldev test)
• The new code is not generating bytecode or M-x checkdoc warnings
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• You've updated the docs (when adding new project types, configuration options, commands, etc)

Thanks!