Add IWYU export pragmas to protect internal headers (#756)

Include What You Use tries to add the internal nanobind headers to files using nanobind.h. Tell it that it should rely on nanobind.h to export all of the internal symbols using the `export` pragma.
https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/IWYUPragmas.md#iwyu-pragma-export

Author: adzenith

include/nanobind/nanobind.h

@@ -36,6 +36,7 @@
 #include <new>
 
 // Implementation. The nb_*.h files should only be included through nanobind.h
+// IWYU pragma: begin_exports
 #include "nb_python.h"
 #include "nb_defs.h"
 #include "nb_enums.h"
@@ -52,6 +53,7 @@
 #include "nb_call.h"
 #include "nb_func.h"
 #include "nb_class.h"
+// IWYU pragma: end_exports
 
 #if defined(_MSC_VER)
 #  pragma warning(pop)