[WIP] Add sass maps support

#### TODO

- [x] basic map parsing
- [ ] advanced map parsing
- [x] length
- [x] map-get
- [x] map-merge
- [x] map-remove (3.3 syntax)
- [x] map-keys
- [x] map-values
- [x] map-has-key
- [x] keywords
`Argument->name()` has the `$` variable prefix that needs to be striped. There's probably a nicer way to do this.