fix(packaging): Ensure typing files are included in wheel

The `highlightedtextbox.pyi` file was missing from the distributed
package because a .gitignore rule was excluding it from build.

Also adds the `py.typed` marker file, to ensure the package is correctly identified as typed per PEP 561.

src/.gitignore

@@ -5,5 +5,4 @@ __pycache__/
 *.py[cod]
 *$py.class
 __tmp/*
-*.pyi
-node_modules
+node_modules