[FEATURE] Single-color argument support

[CypherPoet]

Is your feature request related to a problem? Please describe.

Currently each modifier is calling to SwiftUI's Gradient under the hood. Because Gradient needs to be called with an array of more than one color, using one "start" and one "end" color doesn't work:

RoundedRectangle(cornerRadius: 24)
    .animatableAngularGradient(
          startColors: [.systemRed],
          endColors: [.systemBlue],
           completionPercentage: animationCompletion
)

Describe the solution you'd like.

It would be nice to support this by just animating a single Color instead of a Gradient.