Add darker, lighter and contrasting methods to ManimColor

[hchargois]

Overview

This adds 3 new methods on ManimColors: darker(), lighter() and contrasting(), which return new colors.

Motivation

This is useful to derive related colors from a base color, for example to write a function that builds VMobjects with some fill color, a slightly darker border, and an inset text in a contrasting color, all that from a single color parameter instead of 3:

def colored_square(color: ManimColor) -> Square:
    s = Square()
    s.set_fill(color, opacity=1)
    s.set_stroke(color.darker(), width=10)
    s.add(Text("hello", color=color.contrasting()))
    return s

Here's a slightly more exhaustive example:

Generated by this Scene:

def named_swatch(name: str, color: ManimColor) -> VMobject:
    swatch = Rectangle(width=2.25, height=0.6)
    swatch.set_fill(color, opacity=1)
    swatch.set_stroke(width=0)
    swatch.add(Text(name, font_size=24, color=color.contrasting()))
    return swatch


def lighter_darker_swatches(name: str, color: ManimColor) -> VGroup:
    return VGroup(
        named_swatch(".darker(0.4)", color.darker(0.4)),
        named_swatch(".darker()", color.darker()),
        named_swatch(name, color),
        named_swatch(".lighter()", color.lighter()),
        named_swatch(".lighter(0.4)", color.lighter(0.4)),
    ).arrange_submobjects(RIGHT)


class DemoColors(Scene):
    def construct(self):
        swatches = VGroup(
            lighter_darker_swatches("RED", RED),
            lighter_darker_swatches("BLUE", BLUE),
            lighter_darker_swatches("GREEN", GREEN),
            lighter_darker_swatches("GOLD", GOLD),
            lighter_darker_swatches("PURPLE", PURPLE),
            lighter_darker_swatches("GRAY", GRAY),
            lighter_darker_swatches("WHITE", WHITE),
            lighter_darker_swatches("BLACK", BLACK),
        ).arrange_submobjects(DOWN)
        self.add(swatches)