Add `test` command for consolidated npm testing

See merge request !19

Prevent block-editor script from loading on widgets block editor

Closes #22

See merge request !18

Loading the block-editor script on the widgets block editor produces a
`_doing_it_wrong()` notice because the `wp-editor` script should not be
loaded for the widgets block editor, but it is a dependency for this plugin's
block-editor script.

Fixes #22

Fix tests for PHPUnit 9

See merge request !17

Fix missed version bumps in !15

See merge request !16

Update translation textdomain per WP standards

Closes #18

See merge request !15

Closes https://github.com/ethitter/WP-Revisions-Control/pull/5.

Fixes #18

Fix upgrade notice for v1.4

See merge request !14

Introduce native Gutenberg controls

See merge request !12

Requires manual configuration, due to https://github.com/stephenharris/wp-readme-to-markdown/issues/14.