[3.x] Various fixes to C# documentation generator

[raulsntos]

Backports #58721 to 3.x (except for adding support for [theme_item] since it doesn't exist in 3.x).

• Fix references to global constants and members of @GlobalScope
• Log errors for missing members or methods
• Finds referenced members that are derived (they were not found before since it was only looking in the current type)
• Hardcodes special case for _init method, in C# replaced with a reference to the constructor
• Ignores properties with slashes (since they are not declared in C# and can't be referenced)
• Refactor bbcode_to_xml method