Expose ammonia's id_prefix option via id_prefix kwarg

[gghez]

Adds an id_prefix keyword argument to clean() and Cleaner() that maps to
ammonia's Builder::id_prefix,
prepending the given string to every allowed id attribute value. This namespaces
user-generated ids so they cannot collide with the ids already present on the
host page.

nh3.clean(
    '<b id="x">hi</b>',
    attributes={"b": {"id"}},
    id_prefix="user-content-",
)
# -> '<b id="user-content-x">hi</b>'

Semantics mirror ammonia:

• Defaults to None (no prefixing — current behaviour).
• The tag and the id attribute must still be whitelisted via attributes.
• Values that already start with the prefix are left unchanged (no double prefix).

Includes tests for clean() and a reusable Cleaner, a doctest example, updated
type stubs, and docstrings (which feed the autodoc-generated docs).

Closes #133