Icons: Add prepare script to generate source; don't bundle

Alternative to (and revert of) #72299 (e82f668)

As of #71878 (8dff7de), Icons are defined as SVG files under
`src/library/`, and an accompanying `build` script will produce their
corresponding TSX files and an index at `src/library/index.ts`.

This patch reverts the addition of `.npmignore` in #72299. With this
revert, we return to the guarantee that any generated source code WILL
NOT be included in the published NPM package.

Instead, this patch adds a `prepare` script for @wordpress/icons to
trigger the code generation. This script is part of several NPM life
cycles, including installation (`install`, `ci`) and packing (`publish`,
`pack`), and runs in the background:

https://docs.npmjs.com/cli/v11/using-npm/scripts#life-cycle-scripts

This change is motivated by the fact that Gutenberg's workflow for
release NPM packages involves calling `npm version` (via Lerna) on a
fresh repository clone. Since `npm version` involves creating new
commits, the clone needs to be prepared to run pre-commit hooks.
Concretely, linting will fail if import paths to the icon library cannot
be resolved.

A more localised fix would have been to explicitly invoke a build of the
icons package during the package-release workflow (after `ci` and before
`version`), but with this approach based on `prepare` we guarantee the
availability of the generation code in (probably) all situations. In
particular, it eliminates the need to bundle the generated code in the
package (as previously mentioned).

Author: mcsf

packages/icons/.npmignore

@@ -45,6 +45,7 @@
 	},
 	"scripts": {
 		"prelint": "npm run build",
+		"prepare": "npm run build",
 		"build": "node lib/generate-library"
 	}
 }