Create shared todo_button.vue component

What does this MR do?

There is a need for a "dumb" todo-button component. This button could eventually be used in:

• Epics
• MRs
• Issues
• Alert Management
• more future cases

For now, it will be used only for design management (#198439 (closed)).

Once merged, I will create follow-up issues for the above use-cases to utilize this new component.

### Why have a slot?

Some existing uses of a todo button, for example, Epics, use a different button when the sidebar is collapsed. Making this component slot-able means an easier migration: see !39921 (closed) for the MR that migrates the Epics "todo button" to this shared todo_button.vue (still WIP).

EDIT: wontdo for now. This will need to be done eventually - I will create follow-up issue for this work.

MR Breakdown

Part	MR
👉 Create shared todo button	!39931 (merged) (you are here)
Add necessary queries and resolvers to Design Management's Apollo client	!40222 (merged)
Add vue_shared/todo_button.vue to Design Management	!39935 (merged)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team