Currently supported nodes:
- Register as it is one of the simplest value nodes (even numbers are
not that simple with that dot handling).
- Plus, both unary and binary.
- Parenthesis, both nesting and calling.
Note regarding unit tests: it stores data for AST in highlighting in
strings in place of tables because luassert fails to do a good job at
representing big tables. Squashing a bunch of data into a single string
simply yields more readable result.
