Allow escaping arbitrary characters in double-quoted strings

[laniakea64]

Currently just strings delimited with " " or """ """ only support escaping a limited set of characters - https://just.systems/man/en/chapter_30.html

Most other languages have a syntax for escaping arbitrary Unicode characters. Having such syntax in just would make some justfiles easier to work with:

• Not all valid Unicode characters are printable. And even within the set of printable characters, some text editors don't always do a good job of displaying some printable Unicode characters. Representing these characters as sequence of ASCII characters is not subject to such problems.
• If you know the escape sequence of a character you want to use, having escape syntax means you can just type the character rather than needing to find the character in a character map and copy+paste.

Additionally, such syntax would be helpful if the recipe attribute suggested in #647 (comment) gets implemented.

Thinking about just's backwards-compatibility guarantee, maybe the best syntax would be copying Rust's \u{...}, since it allows a flexible and unambiguous number of hex digits?

[casey]

This would be great, and I agree that \u{…} is the ideal syntax. It'll require modifying Parser::parse_string_literal, which will be a bit of work to make it process multi-line escape sequences.