fix: use span for non-clickable emojis

[EtienneLem]

Fixes #309

• No more default onClick handler (undefined props.onClick was already handled)
• When onClick is defined, use <Button> with type="button" (see Add new <Emoji/> component or prop that doesn't use <Button/>? #309 (comment))

[nolanlawson]

Awesome, great choice. 👍