- Combine and simplify string and backtick lexing.
- Allow newlines in strings and backticks.
- Add triple-delimited indented strings and backticks. Common indented literal non-blank line leading whitespace is stripped.
- If a literal newline is escaped, it will be suppressed.
- Backticks starting with `#!` are reserved for a future upgrade.
Unify lexing of backticks, cooked strings, and raw strings. Also allow
newlines in backticks and cooked strings, since I can't think of a reason
not to.
