Update preload paths for post, site and widgets editors

[jsnajdr]

Changes the REST endpoints that get preloaded when displaying a page for post, site and widgets editor, so that they are in sync with what the latest version of Gutenberg to be shipped with Core 6.0 really requests.

This patch finished backport of the gutenberg_optimize_preload_paths filter in Gutenberg, and that filter can be removed after this is merged. Trac ticket: https://core.trac.wordpress.org/ticket/55505

Post Editor
The types and taxonomies requests have context changed from edit to view, because these requests were modified in WordPress/gutenberg#37685.

Adding two new preloads: list of reusable blocks (wp_block post type, loaded from /wp/v2/blocks) and permissions for creating pages (OPTIONS /wp/v2/pages). Both requests are made in the useBlockEditorSettings hook, at the top of the post editor React tree.

Site Editor
Like in Post Editor, modify the types request context from edit to view.

The taxonomies preload, together with four other ones, can be removed because the Site Editor doesn't request that data.

Site Editor also doesn't need to preload /wp/v2/settings. These are used only in specialized blocks like Site Title, and by the useTitle hook which is by no means on critical path.

Widgets Editor
Adds a preload for the /wp/v2/widget-types endpoint requested by the registerLegacyWidgetVariations called during initialization.

[gziolo]

Committed with https://core.trac.wordpress.org/changeset/53094.