perf: move IsGeneratedCodeFile to Cli project and use Path(Span) methods

[TimothyMakkison]

Description

• Move IsGeneratedCodeFile method into CSharpier.Cli as its the only project that calls it (and is .NET only avoiding the need for preprocessors)
• Change the method signature to IsGeneratedCodeFile(ReadOnlySpan<char> filePath), this is subtle, but it lets the methods like Path.GetFileName use the ReadOnlySpan<char> overload so this entire method goes from allocating several substrings to allocating zero objects

Profiling Check of the Csharpier repo

Related Issue

#1898