Add nanobind.source_dir() API to obtain the source path (#840)

This is useful when building with other tools or build systems, like `setuptools`
in my case.

src/__init__.py

@@ -4,6 +4,10 @@
 if sys.version_info < (3, 8):
     raise ImportError("nanobind does not support Python < 3.8.")
 
+def source_dir() -> str:
+    "Return the path to the nanobind source directory."
+    return os.path.join(os.path.abspath(os.path.dirname(__file__)), "src")
+
 def include_dir() -> str:
     "Return the path to the nanobind include directory"
     return os.path.join(os.path.abspath(os.path.dirname(__file__)), "include")
@@ -16,6 +20,7 @@ def cmake_dir() -> str:
 
 __all__ = (
     "__version__",
+    "source_dir",
     "include_dir",
     "cmake_dir",
 )