Fix backwards incompatibility with .NET standard 2.0 by re-adding overloads with string parameters

[NoahStolk]

• Fixes #436
• Updates the code generator to always include overloads with string parameters regardless of the target framework, methods with ReadOnlySpan<char> overloads are still supported
• Adds a .NET Standard 2.0 test library to the sample to make sure this doesn't happen again (I don't know if it's possible to write a unit/integration test for this but that would probably be way better)
• Removes all target directives and ReadOnlySpan<char> usages from ImGui.Manual.cs
• Adds a copy of ImGui.Manual.cs (ImGui.Manual.ReadOnlySpan.cs) which includes ReadOnlySpan<char> overloads

If the tests in the sample project are not needed I'm fine with removing them. Please let me know if I've missed anything. Any feedback is appreciated.

[zaafar]

So on .net6 users have to explicitly pass/convert string to span in order to use span variant?

[NoahStolk]

So on .net6 users have to explicitly pass/convert string to span in order to use span variant?

Well, you already kind of had to do that. ReadOnlySpan<char> doesn't really make a difference for string literals (e.g. ImGui.Text("Hello, world!"); regarding heap allocations, since that string will only allocate once. But it does make sense to use spans for interpolated strings, since those always result in some allocations. If you want to prevent all those allocations you already had to provide a custom InterpolatedStringHandler anyway, and a method accepting that custom InterpolatedStringHandler can just return ReadOnlySpan<char> which is then passed to ImGui.

I don't believe this makes a difference for .NET 6+ users, and even if it does it should only be very small string allocations which are far less important than the entire library just not working properly for multi-targeted libraries supporting .NET Standard 2.0.