feat: add symbols to alerts

[mfontanini]

This is a follow up on #423 that adds symbols before the title of alert type elements. The structure of the theme changed a bit to be able to pack the styles for each alert type into a single type.

This causes the following presentation:

> [!note]
> this is a note

> [!tip]
> this is a tip

> [!important]
> this is important

> [!warning]
> this is warning!

> [!caution]
> this advises caution!

>>> [!note] other title
ez
multiline
>>>

To render like this: