Allow adding a header by typing /h1, /h2 etc

[aristath]

Description

Currently users can add a header by typing ### followed by a space.
This PR adds the same behavior for /h1 to /h6 followed by enter/return.

To test:

Typing /h4 + enter/return should create a new block with level:4.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR.

[github-actions]

Size Change: +36 B (0%)

Total Size: 1.21 MB

Filename	Size	Change
build/block-library/index.js	146 kB	+36 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
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build/annotations/index.js	3.78 kB	0 B
build/api-fetch/index.js	3.45 kB	0 B
build/autop/index.js	2.84 kB	0 B
build/blob/index.js	665 B	0 B
build/block-directory/index.js	8.72 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	130 kB	0 B
build/block-editor/style-rtl.css	11.1 kB	0 B
build/block-editor/style.css	11.1 kB	0 B
build/block-library/editor-rtl.css	8.97 kB	0 B
build/block-library/editor.css	8.97 kB	0 B
build/block-library/style-rtl.css	7.83 kB	0 B
build/block-library/style.css	7.84 kB	0 B
build/block-library/theme-rtl.css	837 B	0 B
build/block-library/theme.css	838 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
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build/blocks/index.js	48.1 kB	0 B
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build/components/style-rtl.css	15.2 kB	0 B
build/components/style.css	15.2 kB	0 B
build/compose/index.js	9.81 kB	0 B
build/core-data/index.js	12.3 kB	0 B
build/data-controls/index.js	772 B	0 B
build/data/index.js	8.77 kB	0 B
build/date/index.js	31.8 kB	0 B
build/deprecated/index.js	768 B	0 B
build/dom-ready/index.js	571 B	0 B
build/dom/index.js	4.46 kB	0 B
build/edit-navigation/index.js	11.2 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	6.4 kB	0 B
build/edit-post/style.css	6.39 kB	0 B
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build/edit-site/style-rtl.css	3.88 kB	0 B
build/edit-site/style.css	3.88 kB	0 B
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build/edit-widgets/style-rtl.css	3.12 kB	0 B
build/edit-widgets/style.css	3.12 kB	0 B
build/editor/editor-styles-rtl.css	480 B	0 B
build/editor/editor-styles.css	482 B	0 B
build/editor/index.js	43.1 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	4.65 kB	0 B
build/escape-html/index.js	735 B	0 B
build/format-library/index.js	7.7 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	2.13 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	3.11 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	5.34 kB	0 B
build/notices/index.js	1.79 kB	0 B
build/nux/index.js	3.42 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	791 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/reusable-blocks/index.js	3.06 kB	0 B
build/rich-text/index.js	13.2 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	1.84 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.22 kB	0 B

_{compressed-size-action}

[aristath]

The failing test seems to be unrelated to this PR.

[courtneyr-dev]

Related: https://core.trac.wordpress.org/ticket/51187

[youknowriad]

LGTM 👍