Syncpack: ban classnames from being reintroduced (#79061)

Gutenberg standardised on `clsx` in 2024 ([#61138]) and removed every
`classnames` declaration across the monorepo. There is currently no
guardrail preventing it from being re-added later — an `npm install
classnames` in any workspace would land silently.

Add a `Banned` version group to `syncpack.config.mjs` that fails
`lint:deps` (and CI) on any declaration of `classnames` in any
package.json. The rule is preventive: no current declarations are
affected.

Picked up as the follow-up suggested in #77950's review.

[#61138]: #61138

Co-authored-by: manzoorwanijk <manzoorwanijk@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>

Author: manzoorwanijk

syncpack.config.mjs

@@ -7,6 +7,12 @@ export default {
 			packages: [ '**' ],
 			isIgnored: true,
 		},
+		{
+			label: 'Banned dependencies that should not be reintroduced.',
+			dependencies: [ 'classnames' ],
+			packages: [ '**' ],
+			isBanned: true,
+		},
 		{
 			label: 'peerDependencies use intentionally wide ranges; only enforce that the ranges are mutually satisfiable.',
 			dependencyTypes: [ 'peer' ],