New Feature: Added support for Radial Gradient

[UnMolDeQuimica]

Overview: What does this pull request change?

Adds support for radial gradients with Cairo renderer.

Motivation and Explanation: Why and how do your changes improve the library?

Currently, ManimCE supports only linear gradients and it supports them in a very clunky way. In this PR I add a small step towards improving how gradients work by adding a GradientType enum and implementing support for radial gradients.

Here's an example of what can be done with these changes:

class GradientVariants(Scene):
    def construct(self):
        gradient = color_gradient([RED, GREEN, BLUE], 3)
        circles = VGroup(*[
            Circle(
                fill_opacity=1,
                fill_color=gradient, 
                stroke_opacity=0,
                gradient_type=gradient_type,
                sheen_direction=RIGHT
            ).scale(2)
            for gradient_type in GradientType
        ])
        circles.arrange(RIGHT, buff=1)
        self.add(circles)
        for circle in circles:
            label = Text(
                circle.gradient_type.name,
                font_size=24
            ).next_to(circle, DOWN)
            self.add(label)

This example is added to the documentation.

Links to added or changed documentation pages

GradientType documentation

Updated VMobject documentation

Further Information and Comments

All tests pass as I made it use the LINEAR gradient type by default to support existing code.

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested

Small note: I did not add tests, just ran existing tests and tried a few combinations

[UnMolDeQuimica]

I recently added another PR with modifications to support the alpha value of a ManimColor inside a VMobject. Combining both functionalities, things like this are possible:

from manim import *


class GradientExample(Scene):
    def construct(self):
        gradient = color_gradient(
            [
                ManimColor("#f7f7b6"),
                ManimColor("#ffff00"),
                ManimColor("#ffff00", alpha=0),
                ManimColor("#ffff00", alpha=0),
                ManimColor("#ffff00", alpha=0),
            ],
            5
        )

        self.add(
            Circle(
                radius=4,
                fill_color=gradient,
                fill_opacity=1,
                gradient_type=GradientType.RADIAL,
                stroke_opacity=0,
            )
        )