Implement roving tabindex on the Custom HTML block toolbar

[diegohaz]

This PR is part of #18619, whose main goal is to implement roving tabindex on the @wordpress/components' Toolbar component and use it on the header and block toolbars so they become a single tab stop as recommended by the WAI-ARIA Toolbar Pattern. Related issues are #15331 and #3383.

This PR implements the roving tabindex method on the Custom HTML block toolbar.

How to test

• Create a Custom HTML block and check if the toolbar is working properly with mouse and keyboard.

[github-actions]

Size Change: +4 B (0%)

Total Size: 1.12 MB

Filename	Size	Change
build/block-library/index.js	129 kB	+4 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.14 kB	0 B
build/annotations/index.js	3.62 kB	0 B
build/api-fetch/index.js	3.4 kB	0 B
build/autop/index.js	2.83 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	7.26 kB	0 B
build/block-directory/style-rtl.css	937 B	0 B
build/block-directory/style.css	937 B	0 B
build/block-editor/index.js	106 kB	0 B
build/block-editor/style-rtl.css	10.7 kB	0 B
build/block-editor/style.css	10.7 kB	0 B
build/block-library/editor-rtl.css	7.86 kB	0 B
build/block-library/editor.css	7.87 kB	0 B
build/block-library/style-rtl.css	8 kB	0 B
build/block-library/style.css	8.01 kB	0 B
build/block-library/theme-rtl.css	730 B	0 B
build/block-library/theme.css	732 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	48.1 kB	0 B
build/components/index.js	195 kB	0 B
build/components/style-rtl.css	15.9 kB	0 B
build/components/style.css	15.8 kB	0 B
build/compose/index.js	9.6 kB	0 B
build/core-data/index.js	11.4 kB	0 B
build/data-controls/index.js	1.29 kB	0 B
build/data/index.js	8.44 kB	0 B
build/date/index.js	5.47 kB	0 B
build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	569 B	0 B
build/dom/index.js	3.17 kB	0 B
build/edit-navigation/index.js	8.26 kB	0 B
build/edit-navigation/style-rtl.css	1.02 kB	0 B
build/edit-navigation/style.css	1.02 kB	0 B
build/edit-post/index.js	303 kB	0 B
build/edit-post/style-rtl.css	5.5 kB	0 B
build/edit-post/style.css	5.5 kB	0 B
build/edit-site/index.js	16.6 kB	0 B
build/edit-site/style-rtl.css	3.03 kB	0 B
build/edit-site/style.css	3.03 kB	0 B
build/edit-widgets/index.js	9.33 kB	0 B
build/edit-widgets/style-rtl.css	2.43 kB	0 B
build/edit-widgets/style.css	2.43 kB	0 B
build/editor/editor-styles-rtl.css	468 B	0 B
build/editor/editor-styles.css	469 B	0 B
build/editor/index.js	44.8 kB	0 B
build/editor/style-rtl.css	3.8 kB	0 B
build/editor/style.css	3.8 kB	0 B
build/element/index.js	4.65 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	7.72 kB	0 B
build/format-library/style-rtl.css	542 B	0 B
build/format-library/style.css	543 B	0 B
build/hooks/index.js	2.13 kB	0 B
build/html-entities/index.js	622 B	0 B
build/i18n/index.js	3.56 kB	0 B
build/is-shallow-equal/index.js	711 B	0 B
build/keyboard-shortcuts/index.js	2.51 kB	0 B
build/keycodes/index.js	1.94 kB	0 B
build/list-reusable-blocks/index.js	3.12 kB	0 B
build/list-reusable-blocks/style-rtl.css	446 B	0 B
build/list-reusable-blocks/style.css	447 B	0 B
build/media-utils/index.js	5.29 kB	0 B
build/notices/index.js	1.79 kB	0 B
build/nux/index.js	3.4 kB	0 B
build/nux/style-rtl.css	662 B	0 B
build/nux/style.css	657 B	0 B
build/plugins/index.js	2.56 kB	0 B
build/primitives/index.js	1.5 kB	0 B
build/priority-queue/index.js	788 B	0 B
build/redux-routine/index.js	2.85 kB	0 B
build/rich-text/index.js	14 kB	0 B
build/server-side-render/index.js	2.68 kB	0 B
build/shortcode/index.js	1.7 kB	0 B
build/token-list/index.js	1.28 kB	0 B
build/url/index.js	4.06 kB	0 B
build/viewport/index.js	1.85 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

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