Tiled gallery & slider blocks: conditionally transform shortcodes

`[gallery]` shortcode transform is shared between the core gallery Tiled gallery and Slideshow blocks.

Whichever has higher priority set, happens to win (~currently Tiled gallery wins~ update: shortcode transform from the Tiled gallery block was [temporarily removed](https://github.com/Automattic/wp-calypso/pull/30756)).

We should conditionally transform to Tiled gallery and Slideshow blocks only when valid `layout` attribute is set and let core gallery take care of the transform otherwise.

### Problem / blocker

Core doesn't currently support using `isMatch` in shortcode transforms (https://github.com/WordPress/gutenberg/issues/10674 and I don't see a way to divert transformation to some other block conditionally.

**The Best way forward seems to get `isMatch` fixed in core.**

`isMatch` could be something like this for Tiled gallery block:
```js
isMatch: ( { layout } ) => LAYOUT_STYLES.map( style => style.name ).includes( layout )
```

...and for the slideshow block:
```js
isMatch: ( { layout } ) => layout === 'slideshow'
```

Start by reviewing the shared [gallery] shortcode transforms for the Tiled Gallery, Slideshow, and core gallery blocks, then read Gutenberg issue #10674 about isMatch support. Done means valid layout values select the matching block while other gallery shortcodes remain available to core gallery transformation.

Written by the indexing model from the issue text.