Block Editor: Update BEM syntax to CSS modifer guidelines

[aduth]

Previously: #17846 (comment), #16790 (comment)

This pull request seeks to update incorrect usage of BEM modifier syntax to use the is--based modifier syntax as recommended in the CSS naming guidelines.

This impacts two components:

• LinkControl
• ResponveBlockControl

In the latter case, the implementation has been revised as mentioned at #16790 (comment) to unify the application of a single is-responsive modifier class.

Open Question: There was a hidden attribute included previously in ResponsiveBlockControl which would never be applied given how the logic flowed. The changes here seek only to preserve the current behavior. If it needs to be applied, I can restore it as appropriate.

Testing Instructions:

Repeat testing instructions from #17846 and #16790, verifying that there are no regressions.

Ensure tests pass:

npm test

[chrisvanpatten]

It's a bummer that this requires the :not pseudo-selector. I suppose it's not particularly an issue here, but selectors that might potentially have several modifiers will need extra care; scoping a selector for a component's "default" state could require multiple :not checks that could get difficult to manage. Hopefully that's a relatively rare case though.

[chrisvanpatten]

(And I suppose it's more likely to be the case in tests like this; it's probably possible to architect without it being a concern in the CSS itself, and especially if some of the Emotion explorations move forward.)

[aduth]

Yeah, I see this as mostly a use-case we'd want to isolate within tests, where we're explicitly wanting to distinguish between the default and the variant.

In real-world application, we should typically want to treat the variant as something of an additive layer upon the default, even to the extent of unsetting/overriding properties from those defaults, vs. ever feeling the need to use :not( .is-variant ) in CSS. If we found ourselves needing to use :not, it probably raises a bigger question of whether we can really consider there to be a default (vs. other named variants, or treating them as wholly different elements).