Bring implementation of markdown toolbar to core

[dsevillamartin]

We might want to consider bringing the base implementation into core. That way, other extensions can add buttons for things like BBCode without having flarum/markdown enabled.

- flarum/markdown#8 (comment)

Perhaps also a way to replace the buttons shown based on a user preference (BBCode vs Markdown vs other, extensible), but with the option to always show as well (custom bbcode extensions & their buttons, for example).

Will probably want button groups as well... not necessarily a priority but it would be nice.