Add a new setting "skip-comments", which defaults to true. If unset,
this causes lines internal to a non-shebang recipe beginning with the
character '#' (including '#!' internal to a non-shebang recipe; that is,
any such instances occurring after the first line of a recipe) to be
treated as comments of the justfile itself. They will not be echoed to
stderr when the recipe executes.
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`.
