managedBridgeMmForwarder function
A nitrogen link managed .mm forwarder that wraps the generated C++
bridge so SPM compiles it as Obj-C++ and links the C ABI symbols.
relativePath is the relative path from the forwarder to .bridge.g.cpp.
Implementation
String managedBridgeMmForwarder(String relativePath) =>
'// Generated by nitrogen link — do not edit.\n'
'#import <Foundation/Foundation.h>\n'
'#include "$relativePath"\n';