Insertion indicator: render after last block if none is specified

[ellatrix]

Description

As discussed in #25849 (comment), it's not ideal to store appender nodes and to use them for positioning insertion indicators, as these appenders may not always be shown (and perhaps go away in the future). If a block is to be inserted at the end of a block list, we should take the last block and show an indicator after it.

How has this been tested?

Scroll to the bottom of the page without selecting anything and open the inserter. Hover over a block type and you should see the indicator. Do the same with nested blocks: select the last nested block and open the inserter.

Screenshots

Types of changes

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: +49 B (0%)

Total Size: 1.19 MB

Filename	Size	Change
build/block-editor/index.js	128 kB	-42 B (0%)
build/components/index.js	172 kB	+91 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.14 kB	0 B
build/annotations/index.js	3.8 kB	0 B
build/api-fetch/index.js	3.42 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/style-rtl.css	11.2 kB	0 B
build/block-editor/style.css	11.2 kB	0 B
build/block-library/editor-rtl.css	8.88 kB	0 B
build/block-library/editor.css	8.88 kB	0 B
build/block-library/index.js	149 kB	0 B
build/block-library/style-rtl.css	8.34 kB	0 B
build/block-library/style.css	8.34 kB	0 B
build/block-library/theme-rtl.css	789 B	0 B
build/block-library/theme.css	790 B	0 B
build/block-serialization-default-parser/index.js	1.88 kB	0 B
build/block-serialization-spec-parser/index.js	3.06 kB	0 B
build/blocks/index.js	48.1 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	9.95 kB	0 B
build/core-data/index.js	15.2 kB	0 B
build/data-controls/index.js	827 B	0 B
build/data/index.js	8.98 kB	0 B
build/date/index.js	11.2 kB	0 B
build/deprecated/index.js	768 B	0 B
build/dom-ready/index.js	571 B	0 B
build/dom/index.js	4.95 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	6.49 kB	0 B
build/edit-post/style.css	6.47 kB	0 B
build/edit-site/index.js	24.1 kB	0 B
build/edit-site/style-rtl.css	4.06 kB	0 B
build/edit-site/style.css	4.06 kB	0 B
build/edit-widgets/index.js	26.3 kB	0 B
build/edit-widgets/style-rtl.css	3.13 kB	0 B
build/edit-widgets/style.css	3.13 kB	0 B
build/editor/editor-styles-rtl.css	476 B	0 B
build/editor/editor-styles.css	478 B	0 B
build/editor/index.js	43.6 kB	0 B
build/editor/style-rtl.css	3.85 kB	0 B
build/editor/style.css	3.84 kB	0 B
build/element/index.js	4.63 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	2.27 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	698 B	0 B
build/keyboard-shortcuts/index.js	2.54 kB	0 B
build/keycodes/index.js	1.93 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	1.82 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	2.84 kB	0 B
build/reusable-blocks/index.js	2.92 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	2.84 kB	0 B
build/viewport/index.js	1.86 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.22 kB	0 B

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