Use DefaultPathSeparator char instead of DefaultPathSeparatorString

[iSazonov]

PR Summary

Replace string operations on char ones by using overloads like StartsWith(char) and EndsWith(char).

It speeds up path operations in filesystem provider and globbing.

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[anmenaga]

I'm curious, what was the reason why Path Separator was a string (rather than a char)?

[SteveL-MSFT]

@anmenaga .NET Framework only has overload for string, .NET Core added overload for char