Only find files with .sln extension

[Roald87]

Currently if there is a file which is called .slnbak then it can't find a single .sln file and thus it will return an empty sln path.

[Roald87]

It sometimes finds multiple files due to

https://docs.microsoft.com/en-us/dotnet/api/system.io.directory.enumeratefiles?view=netcore-3.1

If the specified extension is exactly three characters long, the method returns files with extensions that begin with the specified extension. For example, "*.xls" returns both "book.xls" and "book.xlsx".