Separator: Add support for opacity controls

[scruffian]

What problem does this address?

The opinionated styles for the separator block set an opacity: https://github.com/WordPress/gutenberg/blob/trunk/packages/block-library/src/separator/theme.scss

It would be good to have these same controls in the block itself, so that they can be controlled by theme.json.

[glendaviesnz]

Playing around with converting the block to use color supports as the color picker for these block supports now includes an opacity setting.

[nanu-c]

i would love to set the opacity for this block in theme.json but opacity is not part of the reference. Should i open an new issue?

[glendaviesnz]

i would love to set the opacity for this block in theme.json but opacity is not part of the reference. Should i open an new issue?

Yes, a new issue is probably the best for this, thanks.