Reject first-segment colons when validating relative URI paths

[ac1982]

Fixes #91's validation case.

uri_reference('.://').is_valid() currently returns True, and Validator().check_validity_of('path') accepts the same invalid relative reference. RFC 3986 §4.2 forbids a colon in the first path segment of a reference without a scheme.

Apply that contextual rule in both the configured validator and the legacy URI validation methods. Parsing remains permissive, consistent with the existing separation of parsing and validation. Standalone path_is_valid() keeps its component-only behavior, since a colon is valid in a rootless path when a scheme is present. Legal references such as urn:a:b, ./a:b, /a:b, and percent-encoded colons remain valid.

Validation on Linux / Python 3.12:

• The new tests fail on the base: 5 failed, 11 passed.
• Full suite: 2,930 passed, with 100% coverage (16 new cases).
• All pre-commit checks, HTML documentation build, 57 doctests, and wheel/sdist builds passed.
• Project typing-coverage check: 76.87%, above the required 75%.

This branch is based directly on main and is independent of #159.