Editor: Improves layout block support in wp_get_layout_style().

This commit merges the remaining changes from [WordPress/gutenberg#40875 Gutenberg PR 40875]. It's Part 2 (see [54162] for Part 1) of a layout improvement initiative and targets `wp_get_layout_style()` in `layout.php`.

Context:
The overall initiative is to improve layout block support:
>to use centralised layout definitions, output base layout styles from global styles, introduce a layout type semantic classname, reduce duplication of container class and style tag output, and fix blockGap at the block level in global styles.

Changes include:
* Adding an optional parameter `$block_spacing` to `wp_get_layout_style()` for setting a custom spacing on the block.
* Adding handle for the block spacing.
* Using the style engine to to enqueue and render layout styles via `wp_style_engine_get_stylesheet_from_css_rules()`.
* Introduces a new test file for `wp_get_layout_style()`.

Follow-up to [54162], [54160], [54159], [53421], [52380], [53085], [52069].

Props andrewserong, isabel_brison, costdev, hellofromTonya.
See #56467.
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git-svn-id: http://core.svn.wordpress.org/trunk@53833 1a063a9b-81f0-0310-95a4-ce76da25c4cd