Snackbar: Fix __unstableHTML support

[ntsekouras]

What?

Fixes: #56767

I added the exact same handling we have from Notice compoenent.

Testing Instructions

In dev tools run:

wp.data
	.dispatch( 'core/notices' )
	.createNotice(
		'warning',
		'<strong>Some</strong> message',
		{
			type: 'snackbar',
			isDismissible: true,
			__unstableHTML: true,
		}
	);

[github-actions]

Flaky tests detected in eabea47.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/7210753452
📝 Reported issues:

• [Flaky Test] should display the "Manage Fonts" icon #56776 in /test/e2e/specs/site-editor/font-library.spec.js

[mirka]

Can I have confirmation from somebody (@WordPress/gutenberg-core) that this is actually needed in Snackbar?

I'm asking because:

1. Despite this __unstableHTML prop being present in the Storybook docs, it's more like a TypeScript mistake and we've never actually shipped this functionality in Snackbar. So although we currently aren't bound by a back-compat contract, we would be bound to it if were to ship it now. In this case we shouldn't prefix it with __unstable anymore, and we should document it properly, warning about its dangers.
2. When we look at the original PR that adds __unstableHTML functionality to the Notice component (which our TS types erroneously pull from), they seem to be quite reluctant about it, saying that it's not ideal and is intended to be "compat-only".

I'm ok with adding it if it is indeed necessary, but if not, we can just fix the TS types.

[ntsekouras]

Good questions!

it's more like a TypeScript mistake

I saw that, but I thought it was just an omission in the implementation and not a TS mistake.

I'll cc @mcsf .

[mcsf]

Thanks for digging out the history; I had forgotten about those discussions from 2018. :)

With all the context laid out, I'm leaning towards not touching snackbars, even if that means consumers will face inconsistencies between them and regular notices.

I don't know if this begs for a proper element-interpolation solution to be brought to notices and snackbars — or if we should first discuss the purpose of notices, and thus what consumers ideally should and shouldn't be able to do.