Add fold markers to various syntax

[Altreus]

Not sure about opening an issue about folding since the issue template mentions it, but I can't see anything that actually implements any.

Vim help mentions this:

SYNTAX						*fold-syntax*

A fold is defined by syntax items that have the "fold" argument. |:syn-fold|

The fold level is defined by nesting folds.  The nesting of folds is limited
with 'foldnestmax'.

Presumably the implication is that one can add syntax items, or add flags to existing ones, to specify the start and end of folds.

I would like to fold POD6 and code blocks, but neither currently seems to be set up by default; in fact it looks like there are no folds in any of my raku source files.

Sadly I have zero experience with vim syntax files or I'd sort it out myself