Add conditional expressions of the form:
foo := if lhs == rhs { then } else { otherwise }
`lhs`, `rhs`, `then`, and `otherwise` are all arbitrary expressions, and
can recursively include other conditionals. Conditionals short-circuit,
so the branch not taken isn't evaluated.
It is also possible to test for inequality with `==`.
If an environment variable exists with the same key as a variable from a
`.env` file, skip the variable from the `.env` file in favor fo the key
from the environment.
Just's first parser performed both parsing, i.e the transformation of a
token stream according to the language grammar, and a number of consistency
checks and analysis passes.
This made parsing and analysis quite complex, so this diff introduces a
new, much cleaner `Parser`, and moves existing analysis into a dedicated
`Analyzer`.
