Expose `ScriptItem`

[schloerke]

When making wrappers around HTMLDependency, is is useful to have the helper types. Ex: _jsx.py uses ScriptItem.

Issues:

• If a dictionary is passed in as a variable, the type is of a dict[str, str].
• If a TypedDict is used to try to get around it, then it is a type mismatch.

Both situations would be solvable if the Type was exposed