Add py.typed (#421)

This pr adds py.typed to enable linters.

Signed-off-by: Ubospica <[email protected]>

Author: Ubospica

pyproject.toml

@@ -78,6 +78,7 @@ sdist.include = [
   "/cpp/**/*.h",
   "/include/**/*",
   "/python/xgrammar/**/*.py",
+  "/python/xgrammar/py.typed",
 
   # Third party files
   "/3rdparty/**/*",