Handling label types between VTL, Markdown and simple text

There are different types of label in Pogues:

1. simple text
2. Markdown text
3. VTL expression

The first and second could be presented as a single option (simple text being valid Markdown) but are distinct from the third one.

We need to manage those different types. A previous idea was to ask users to consider simple text / markdown as a VTL string (between `"`), but some users want to still use simple text as input for their labels.

So we need to handle that explicitly, with a UI for choosing the type of labels (Markdown or VTL) for each label fields.

__This need to be validated__