Add a way to add inline text background color

[iamtakashi]

It'd be nice to be able to add an inline text background color to any text. I've seen it's being called "highlight", but I'm not sure the proper name :)

Example:

The text can be multi-lined, and a little detail that makes this nicer is to have side paddings so that the background doesn't get cut off immediately at where the text begins/ends. An old CSS Tricks that talks about this.

Previously: #16014

[paaljoachim]

Associated with
"Bring colors by the block for some blocks"
#20197

[dingo-d]

It would also be cool to add the gradient to the highlighted text in the RichText component. Currently, only the color can be added, either via inline color: style on has-inline-color span element or by referencing the color name registered in the global colors.

Since the inline color already uses a span to format it, and in order to apply the gradient to the text we'd need to add it as a background with background-clip, this shouldn't be that hard to implement.