Fix: canInsertBlockType() returns false positive

[delowardev]

Description

For those blocks doesn't support multiple, wp.data.select( 'core/block-editor' ).canInsertBlockType( 'core/more' ) should return false if that block already inserted.

How has this been tested?

Screenshots

Peek.2021-12-20.14-33.mp4

Types of changes

Bugfix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• 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 (please manually search all *.native.js files for terms that need renaming or removal).