Template Part: Add extra validation when loading template parts

[noisysocks]

Description

Adds some extra validation when the Template Part block loads template part files.

How has this been tested?

1. Enable the Gutenberg full site editing experiment.

2. Add the following files to your active theme's folder:

   /block-templates/single.html:

   <!-- wp:template-part {"slug":"header","theme":"twentytwenty"} /-->
   <!-- wp:paragraph -->
   <p>Single Template</p>
   <!-- /wp:paragraph -->

   /block-template-parts/header.html:

   <!-- wp:paragraph -->
   <p>Header TwentyTwenty Template Part</p>
   <!-- /wp:paragraph -->

3. Create a post and insert a Template Part block with slug set to header and theme set to twentytwenty

4. View the post

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: 0 B

Total Size: 840 kB

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.02 kB	0 B
build/annotations/index.js	3.62 kB	0 B
build/api-fetch/index.js	4.01 kB	0 B
build/autop/index.js	2.82 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	6.24 kB	0 B
build/block-directory/style-rtl.css	760 B	0 B
build/block-directory/style.css	760 B	0 B
build/block-editor/index.js	105 kB	0 B
build/block-editor/style-rtl.css	10.2 kB	0 B
build/block-editor/style.css	10.2 kB	0 B
build/block-library/editor-rtl.css	7.1 kB	0 B
build/block-library/editor.css	7.1 kB	0 B
build/block-library/index.js	112 kB	0 B
build/block-library/style-rtl.css	7.14 kB	0 B
build/block-library/style.css	7.15 kB	0 B
build/block-library/theme-rtl.css	683 B	0 B
build/block-library/theme.css	685 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	57.7 kB	0 B
build/components/index.js	198 kB	0 B
build/components/style-rtl.css	16.7 kB	0 B
build/components/style.css	16.7 kB	0 B
build/compose/index.js	6.66 kB	0 B
build/core-data/index.js	11.1 kB	0 B
build/data-controls/index.js	1.25 kB	0 B
build/data/index.js	8.43 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	3.1 kB	0 B
build/edit-navigation/index.js	3.1 kB	0 B
build/edit-navigation/style-rtl.css	279 B	0 B
build/edit-navigation/style.css	280 B	0 B
build/edit-post/index.js	27.8 kB	0 B
build/edit-post/style-rtl.css	12.3 kB	0 B
build/edit-post/style.css	12.3 kB	0 B
build/edit-site/index.js	10.4 kB	0 B
build/edit-site/style-rtl.css	5.02 kB	0 B
build/edit-site/style.css	5.02 kB	0 B
build/edit-widgets/index.js	7.53 kB	0 B
build/edit-widgets/style-rtl.css	4.65 kB	0 B
build/edit-widgets/style.css	4.64 kB	0 B
build/editor/editor-styles-rtl.css	428 B	0 B
build/editor/editor-styles.css	431 B	0 B
build/editor/index.js	43.3 kB	0 B
build/editor/style-rtl.css	3.48 kB	0 B
build/editor/style.css	3.47 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	7.32 kB	0 B
build/format-library/style-rtl.css	502 B	0 B
build/format-library/style.css	502 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	710 B	0 B
build/keyboard-shortcuts/index.js	2.51 kB	0 B
build/keycodes/index.js	1.91 kB	0 B
build/list-reusable-blocks/index.js	3.12 kB	0 B
build/list-reusable-blocks/style-rtl.css	226 B	0 B
build/list-reusable-blocks/style.css	226 B	0 B
build/media-utils/index.js	5.28 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	616 B	0 B
build/nux/style.css	613 B	0 B
build/plugins/index.js	2.67 kB	0 B
build/primitives/index.js	1.49 kB	0 B
build/priority-queue/index.js	788 B	0 B
build/redux-routine/index.js	2.84 kB	0 B
build/rich-text/index.js	14.8 kB	0 B
build/server-side-render/index.js	2.67 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	4.02 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	1.17 kB	0 B

_{compressed-size-action}

[talldan]

Looks good 🚢