feat(core): find todo list based on 'checkbox' search query (#13982)

This feature enhances the /slash command by allowing users to search for
'checkbox' and have the to-do list item show up as a result. Users come
from different systems and environments, and some may use the name
'checkbox' but be confused as they cannot find it in the search menu.

This is achieved by adding a `searchAlias` property on the to-do list
item block that contains the string `checkbox`.

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added search-alias support for slash menu items so entries can be
found by alternative terms.
* To-do List entry now includes "checkbox" as an additional searchable
alias to improve discoverability.
* Slash menu search results updated to reflect alias-driven matches
(additional item appears when searching).

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: DarkSky <25152247+darkskygit@users.noreply.github.com>