Support any string.

Right now we don't allow a couple of things in strings.

• Strings can't have newlines in the end.
• The first line of the string can't be indented.

I think the plan is to add quoted strings.

inline: "foo bar # not a comment."
multi-line: """
  indented!
less indented

"""

The inline quoted string is technically enough to get us any string, but it would be nice to have a multi-line syntax.