editor: Add word count plugin to RichEditor toolbar

[Samk13]

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Description

• This is useful when the input field has a character count limit, allowing users to see the current character count.
• Related to https://github.com/inveniosoftware/invenio-requests/pull/391
• Partially closes: https://github.com/inveniosoftware/invenio-app-rdm/issues/2731

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• [ ] I'm aware of the code of conduct.
• [ ] I've created logical separate commits and followed the commit message format.
• [ ] I've added relevant test cases.
• [ ] I've added relevant documentation.
• [ ] I've marked translation strings.
• [ ] I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• [ ] I've NOT included third-party code (copy/pasted source code or new dependencies).
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Frontend

• [ ] I've followed the CSS/JS and React guidelines.
• [ ] I've followed the web accessibility guidelines.
• [ ] I've followed the user interface guidelines.

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