Doesn't layout contents into a box anymore, instead layouting inline in the parent context. Also makes axis inferring for center alignents smarter (just because I had fun doing it). It's unsure whether we want to keep it because it might be confusing.
- In addition to syntax trees there are now `Value`s, which syntax trees can be evaluated into (e.g. the tree is `5+5` and the value is `10`)
- Parsing is completely pure, function calls are not parsed into nodes, but into simple call expressions, which are resolved later
- Functions aren't dynamic nodes anymore, but simply functions which receive their arguments as a table and the layouting context
- Functions may return any `Value`
- Layouting is powered by functions which return the new `Commands` value, which informs the layouting engine what to do
- When a function returns a non-`Commands` value, the layouter simply dumps the value into the document in monospace
Previously they were passed as strings to the function parser, now they are parsed and then passed as trees to the function. This allows making bodies sugar for a last content argument. While it removes some flexibility allowing function to parse arbitrary syntaxes in their bodies, these can be modelled as (raw) string arguments.
