Modify the lexer to keep track of opening `({[` and closing `]})` delimiters.
When the lexer would emit an eol or indent outside of a recipe when there
is at least one open delimiter, emit a whitespace token instead.
This allows expressions to be split on multiple lines, like so:
x := if 'a' == 'b' {
'x'
} else {
'y'
}
This does not work inside of recipe body interpolations, although this
restriction might relaxed in the future.
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 `==`.
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`.
