[Blocks] Paragraph and Heading: add gradient support

[MaggieCabrera]

What?

Adds gradient supports for paragraph and heading blocks
Closes #30982
Part of #41113

Why?

Before we had a way to hide block settings from the sidebar we had to be careful about what we were exposing in the UI, that is not the case anymore and we can add supports as users need them more freely without fearing UI clutter.

How?

Just added the support to block.json :D

Testing Instructions

Open the editor and change the background of the block to a gradient! Then check the frontend too.

Screenshots or screencast

Tested on emptytheme:

/cc @WordPress/block-themers

[bgardner]

Considering the option for Gradient exists with the Code, Pullquote, and Quote blocks, this makes sense (and moves us closer to overall block settings consistency).

[scruffian]

LGTM

[briceduclos]

@MaggieCabrera Thanks! It would be great also to add gradient support to the Heading block.

[carolinan]

Maybe we can keep the issue open and update the title because I would hate to loose the suggestion for gradient texts 😆

[MaggieCabrera]

There you go!

[bph]

Added label needs dev note to add it to a list of other changes for enabling and disabling features in block.json / theme.json.

[bgardner]

@carolinan Gradient texts... like so? https://bg.design/bright-mode/

[MaggieCabrera]

@carolinan Gradient texts... like so? https://bg.design/bright-mode/

Like #30982

Right now we are checking for accesibility issues with text colors, right? For text gradients it would be the same, but testing all the colors involved in the gradient, am I right?

[bph]

removed needs dev note - as it's only a UI change. Disabling duotone is already available in 6.0