[EuiIcon] Add checklist glyph

[andreadelrio]

Summary

Adds editorChecklist glyph which we need for the Controls projects in Dashboard. Below is a screenshot of the control type we're trying to represent.

Design

Also, replaced custom icon in MarkdownEditor with this new icon

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
  - [ ] Props have proper autodocs and **[playground toggles](https://github.com/elastic/eui/blob/main/wiki/documentation-guidelines.md#adding-playground-toggles)**
• Added documentation
  - [ ] Checked Code Sandbox works for any docs examples
  - [ ] Added or updated **jest and [cypress](https://github.com/elastic/eui/blob/main/wiki/cypress-testing.md) tests**
  - [ ] Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[cchaos]

Quick suggestion, this looks just like a simple checklist. How about naming it that instead?

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5705/

[elizabetdev]

Thanks, @andreadelrio. It looks great! 🎉

One thing that I noticed is that visually the icons are more similar to the list icons in editor controls. In common they have three list items. So I think we can move the icon there and update the design to make it more similar to these icons (like having round rectangles)

Also the EuiMarkdownEditor is using a custom icon that lives in src/components/markdown_editor/icons/markdown_checkmark.tsx, do you mind to replace it with this new one and deleting the old one markdown_checkmark?

Also, don't forget to add the new icon to our Figma library! Thanks! 😄

[andreadelrio]

@miukimiu great suggestions! implemented them all and made the icon's rectangles rounded.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5705/

[elizabetdev]

Thanks, @andreadelrio! LGTM! 🎉

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_5705/