fix(parser): fix some escape sequences in strings and indentation in block strings

[goto-bus-stop]

Follow-up to #638, fixing conversion of StringValue nodes to Rust strings.

Fixes #609
Fixes #611

This handles

• escape sequences in single-quoted strings and
• indentation in block strings Block strings are parsed like non-block strings #609
• wrong behaviours when a string ends with an escaped " Incorrect parsing of string with trailing \" #611

This does not handle these unicode escape issues:

• Panic with surrogate code points in EscapedUnicode #608
• EscapedUnicode in draft spec supports \u{12345678} syntax #640

[SimonSapin]

Looks great!

[SimonSapin]

The above is fine. It can be marginally nicer with split_at and strip_prefix:

Suggested change

	let line = &self.input[..index];
	let rest = match self.input.get(index..=index + 1) {
	Some("\r\n") => &self.input[index + 2..],
	_ => &self.input[index + 1..],
	};
	self.input = rest;
	let (line, rest) = self.input.split_at(index);
	// `rest` starts with \r or \n
	self.input = rest.strip_prefix("\r\n").unwrap_or_else(|| &rest[1..])

[goto-bus-stop]

my personal feeling is that split_at looks nicer but the manual match is more obviously correct despite being a bit noisy... though as i look back and forth i don't feel strongly anymore.

[lrlna]

\o/