Allow a letter before a string literal

- Python has `r"..."` syntax for raw strings. Rust has `r#"..."#`.
- Python has `f"..."` syntax for format strings.
- Rust has `b"..."` syntax for byte strings and `c"..."` for C strings.
- Kotlin has `return` syntax to allow `return` to be used as an identifier. This might be better written i"return".

Allow a letter before a string literal in Welly. Decide on the meanings later.