Add colour presenter aliases

[OskarEichler]

Rainbow's top-level API aliases colour to color, but the returned enabled and disabled presenter objects do not. This makes Rainbow('text').colour(:red) unavailable even though the spelling is advertised by the factory API, and disabled-mode parity is missing too.

Add the same alias to Presenter and NullPresenter.

Verification:

• release/master fail the isolated enabled/disabled reflection and invocation model; this branch passes all four checks on Ruby 3.2.11 and 4.0.6
• full suite: 220 examples / 0 failures on both Rubies
• isolated rainbow_spec no longer depends on another spec's global String monkey patch
• 15-file RuboCop run: 0 offenses on both Rubies
• rebuilt 14-file gem passes the packaged model

No API break.