P2-814 Implement appender according to design

[hansjovis]

Summary

This PR can be summarized in the following changelog entry:

• [@yoast/schema-blocks] Fixes the styling of the block appender used in the InnerBlocks block instruction.

Relevant technical choices:

Test instructions

This PR can be tested by following these steps:

• Create a post.
• Add a job posting block.
• Select the job posting block (if it is not selected already).
• You should see a block appender at the bottom of the job posting.
  • It should look like the block appender in the design.
  • Note: It may look slightly different based on the theme you have installed on your website. E.g. a different background and/or font. All in all the styling should look consistent with the rest of the theme.
• Make sure that the block appender works as expected.
  • E.g. remove the job description from the job posting and re-add it via the block appender.

Impact check

• This PR affects the following parts of the plugin, which may require extra testing:
  • The location and base salary inner blocks of the job posting, which have inner blocks to be able to add variations (e.g. remote location or office location), should not have an appender.

UI changes

• [x] This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Quality assurance

• [x] I have tested this code to the best of my abilities
• [x] I have added unittests to verify the code works as intended

Fixes #

[johannadevos]

Acceptance: ✅

• [x] The appender becomes visible when the Job Posting block is selected.
• [x] It is possible to add a block through the appender.
• [x] The styling looks good.
• [x] The variation blocks don't have an appender.
• [ ] It is a known issue that clicking on appender selects the parent block: https://yoast.atlassian.net/browse/P2-782.