Add default block widths to the full-site editor

[kjellr]

Fixes #26630.

The full-site editor does not currently set the default widths for blocks, leaving all blocks to appear full-width unless a theme specifically adds a max-width.

This PR brings the max-width rules from edit-post into edit-site too. I created a mixin for them to avoid duplicating code. I also moved the wide-content width (1100px) into a new variable since it was used in multiple places.

Testing

1. Activate a block-based theme that does not set the default width for blocks in its editor styles.*
2. Add default, wide, and full-width blocks in the full site editor. Verify that they appear similar to the way they do in the post editor.

*You can use Twenty Twenty-One Blocks if you remove these lines.

Before	After

[github-actions]

Size Change: +72 B (0%)

Total Size: 1.21 MB

Filename	Size	Change
build/edit-site/style-rtl.css	3.95 kB	+39 B (0%)
build/edit-site/style.css	3.95 kB	+41 B (1%)
build/editor/editor-styles-rtl.css	476 B	-4 B (0%)
build/editor/editor-styles.css	478 B	-4 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.14 kB	0 B
build/annotations/index.js	3.77 kB	0 B
build/api-fetch/index.js	3.42 kB	0 B
build/autop/index.js	2.83 kB	0 B
build/blob/index.js	665 B	0 B
build/block-directory/index.js	8.71 kB	0 B
build/block-directory/style-rtl.css	943 B	0 B
build/block-directory/style.css	942 B	0 B
build/block-editor/index.js	133 kB	0 B
build/block-editor/style-rtl.css	11.2 kB	0 B
build/block-editor/style.css	11.1 kB	0 B
build/block-library/editor-rtl.css	8.91 kB	0 B
build/block-library/editor.css	8.91 kB	0 B
build/block-library/index.js	147 kB	0 B
build/block-library/style-rtl.css	8.1 kB	0 B
build/block-library/style.css	8.1 kB	0 B
build/block-library/theme-rtl.css	792 B	0 B
build/block-library/theme.css	793 B	0 B
build/block-serialization-default-parser/index.js	1.87 kB	0 B
build/block-serialization-spec-parser/index.js	3.06 kB	0 B
build/blocks/index.js	48 kB	0 B
build/components/index.js	171 kB	0 B
build/components/style-rtl.css	15.3 kB	0 B
build/components/style.css	15.3 kB	0 B
build/compose/index.js	9.87 kB	0 B
build/core-data/index.js	14.7 kB	0 B
build/data-controls/index.js	1.12 kB	0 B
build/data/index.js	8.74 kB	0 B
build/date/index.js	31.8 kB	0 B
build/deprecated/index.js	769 B	0 B
build/dom-ready/index.js	571 B	0 B
build/dom/index.js	4.45 kB	0 B
build/edit-navigation/index.js	11.1 kB	0 B
build/edit-navigation/style-rtl.css	881 B	0 B
build/edit-navigation/style.css	885 B	0 B
build/edit-post/index.js	306 kB	0 B
build/edit-post/style-rtl.css	6.43 kB	0 B
build/edit-post/style.css	6.42 kB	0 B
build/edit-site/index.js	22.6 kB	0 B
build/edit-widgets/index.js	26.3 kB	0 B
build/edit-widgets/style-rtl.css	3.16 kB	0 B
build/edit-widgets/style.css	3.16 kB	0 B
build/editor/index.js	42.5 kB	0 B
build/editor/style-rtl.css	3.85 kB	0 B
build/editor/style.css	3.85 kB	0 B
build/element/index.js	4.62 kB	0 B
build/escape-html/index.js	735 B	0 B
build/format-library/index.js	6.86 kB	0 B
build/format-library/style-rtl.css	547 B	0 B
build/format-library/style.css	548 B	0 B
build/hooks/index.js	2.16 kB	0 B
build/html-entities/index.js	623 B	0 B
build/i18n/index.js	3.57 kB	0 B
build/is-shallow-equal/index.js	712 B	0 B
build/keyboard-shortcuts/index.js	2.52 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.1 kB	0 B
build/list-reusable-blocks/style-rtl.css	476 B	0 B
build/list-reusable-blocks/style.css	476 B	0 B
build/media-utils/index.js	5.31 kB	0 B
build/notices/index.js	1.77 kB	0 B
build/nux/index.js	3.4 kB	0 B
build/nux/style-rtl.css	671 B	0 B
build/nux/style.css	668 B	0 B
build/plugins/index.js	2.56 kB	0 B
build/primitives/index.js	1.43 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.84 kB	0 B
build/reusable-blocks/index.js	3.05 kB	0 B
build/rich-text/index.js	13.4 kB	0 B
build/server-side-render/index.js	2.77 kB	0 B
build/shortcode/index.js	1.69 kB	0 B
build/token-list/index.js	1.27 kB	0 B
build/url/index.js	4.06 kB	0 B
build/viewport/index.js	1.83 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.22 kB	0 B

_{compressed-size-action}

[jasmussen]

Yep. Nice.

At first I couldn't reproduce. But turns out I had defined the wide and fullwide widths myself in my editor style. And sure enough, they're not there by default.

[kjellr]

Thanks everyone!