Similar to TextInput component.

• Validation should happen on every keystroke.
• Use validation created in T275113.
• Emit event if input is invalid so that the consuming application can react to it
• Maybe emit event if input is valid?

Error message handling

• Either let the consuming application provide the error message when it receives the error event
• Or have it always provide the validation-error message via a prop and have this component decide when to show it

Storybook:

• The extended number input component will be included in its own Storybook page, under the section "Wikibase components":
• Basic (controllable story displaying a single number input: Controls should allow users to modify the label and placeholder texts)
• All (story displaying all number input states for visual regression testing purposes)