Add trash post type label

[ntsekouras]

Now as we can see from the available post type labels, there is not one for trashing an entity.
Ref here: ​https://developer.wordpress.org/reference/functions/get_post_type_labels/.

In Gutenberg when you trash an entity should show a message from the labels, so as to be translated as well.

Related issue in Gutenberg here: ​https://github.com/WordPress/gutenberg/issues/19144

Added the item_trashed default label to the get_default_labels() method.

This update adds the item_trashed default label to the get_default_labels() method. As per ​#25604 , this should allow ​#25563 to move forward now that the strings are translatable. Note: ​#25563 was reverted, pending this update to core.

Testing Instructions:

1. Manually apply the changes in ​#25563 to a local copy of the Gutenberg plugin.
2. Remove the if statement [ ​https://github.com/WordPress/gutenberg/blob/trunk/packages/editor/src/store/utils/notice-builder.js#L26 here].
3. Instead of noticeMessage = ${ postType.labels.singular_name } trashed.;, use noticeMessage = postType.labels.item_trashed;
4. Re-build Gutenberg - npm run build.
5. Open a post in the editor - On the Document settings pane, click on "Move to trash".
6. Voice Over should read "Post trashed" before redirecting.

Trac ticket: https://core.trac.wordpress.org/ticket/51387

E2E test locally gives the following results:

Test Suites: 6 passed, 6 total
Tests:       15 passed, 15 total
Snapshots:   0 total
Time:        117.714 s, estimated 224 s

Would it be possible to re-run it?

[SergeyBiryukov]

In 55923:

Posts, Post Types: Introduce item_trashed post type label.

This allows the block editor to announce the correct message when an entity is moved to the Trash.

Original issue from Gutenberg repository:

• ​#19144 Wrong notification message shown when an entity is moved to trash

Props james-roberts, ntsekouras, nrqsnchz, afercia, swissspidy, joedolson, talldanwp, SergeyBiryukov.
Fixes #51387.

Thanks for the PR! Merged in r55923.

[stevenlinx]

Added to misc dev note.
draft: ​https://make.wordpress.org/core/?p=106236&preview=1&_ppp=2977223417