[BBT-95] Adds Support for Gradients

[chrishall0]

Description

Fixes #95. This PR adds a GradientPicker component.

Component has been added within the Color component rather than it's own section as this is where it would be within the core Site Editor. In the theme.json scheme the gradient values live within the color options also.

Change Log

• Added StyleGradient Component to existing Color Component to allow users to add a color gradient to site/blocks.
• Added support to disable custom gradients using customGradient theme.json setting
• Added support for default gradients where defaultGradients option is true (true by default)

Steps to test

Default Gradient options should be available with a standard theme.json (i'm using twenty twenty three) unless settings.color.defaultGradients is set to false.

Theme Gradients will need to be added to test those. For example theme.json could be updated - settings.color.gradients

"gradients": 
	[
		{
			"gradient": "linear-gradient(135deg, #9DFF20 0%, #345C00 100%)",
			"name": "Primary Gradient",
			"slug": "primary-gradient"
		}
	]

Setting settings.color.customGradient to false in the theme.json should hide the ability to customise gradients in the Component.

Screenshots/Videos

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

[g-elwell]

Thanks for picking this up @chrishall0 🙌

[chrishall0]

@g-elwell made a small change to the if statement before the render, noticed it wasn't rendering in scenarios where there were no preset gradients available however the tool is still usable to create custom gradients

[g-elwell]

Works great, thanks @chrishall0 !