Multiline dollar escape sequence not handled properly

[darichey]

Describe the bug

The tokenizer does not correctly handle one of the possible escape sequences for $ in multiline strings: ''\$.

Code Snippet to reproduce

fn main() {
    for token in rnix::tokenize(r#"''''\${''"#) {
        println!("{token:?}")
    }
}

Expected behavior

(TOKEN_STRING_START, "''")
(TOKEN_STRING_CONTENT, "''\\${")
(TOKEN_STRING_END, "''")

Actual behavior

(TOKEN_STRING_START, "''")
(TOKEN_STRING_CONTENT, "''\\")
(TOKEN_INTERPOL_START, "${")
(TOKEN_STRING_START, "''")
(TOKEN_ERROR, "")