`is_normalized_whatwg()` method is non-compliant to WHATWG spec

[lo48576]

When the authority is absent, the path is relative, and the path after RFC 3986 normalization result starts with //, then WHATWG URL Standard handles the path as "opaque" and does not touch it during normalization.
However, the current iri-string crate does not support "opaque" paths.
So, foo:.///bar is untouched by WHATWG URL Standard but is_normalized_whatwg() returns false for it.

Possibly I should not say or imply the normalization as compliant to "WHATWG URL Standard", and this method should have different name.

Related to #29.

[lo48576]

Changing method name is breaking change, so at first the method should be deprecated and then removed in next major release.

[lo48576]

Fixed by 07029b8.