Storybook: Add stories for IsolatedBlockEditor

[mirka]

Adds some stories of the basic <IsolatedBlockEditor> for easier exploration of settings, testing of behaviors, and comparison with the collab-ready instances.

cc @johngodley in case you want to dev/test with it too.

To test

Note that standalone builds are currently broken and blocked due to WordPress/gutenberg#35282. To build, you'll need a local copy of the latest Gutenberg repo and replace the @wordpress/keyboard-shortcuts package with it.

// package.json

-  "@wordpress/keyboard-shortcuts": "^3.0.2",
+  "@wordpress/keyboard-shortcuts": "file:../gutenberg/packages/keyboard-shortcuts",

Then run yarn storybook.