fix(syntax/lexer): stop strings at a backslash-escaped newline

The escape arm consumed whatever byte followed the backslash, so a
backslash at the end of a line swallowed the newline and the lexer ran
into the following lines until the next quote, restructuring the rest
of the document into one string token. The newline guard now applies
inside escapes too.

src/syntax/lexer.rs

@@ -103,6 +103,12 @@ impl<'a> Lexer<'a> {
                 }
                 b'\\' => {
                     self.pos += 1;
+                    if self.bytes.get(self.pos) == Some(&b'\n') {
+                        // An escaped newline is not a JSON continuation,
+                        // so the newline still terminates the line.
+                        self.error(start, "unterminated string");
+                        break;
+                    }
                     if self.pos < self.bytes.len() {
                         self.advance_char();
                     }
@@ -259,6 +265,15 @@ mod tests {
         assert_eq!(errors.len(), 1);
     }
 
+    #[test]
+    fn backslash_before_newline_ends_the_string() {
+        let (tokens, errors) = tokenize("\"C:\\\ntrue");
+        assert_eq!(tokens[0].kind, SyntaxKind::Str);
+        assert_eq!(tokens[0].text, "\"C:\\");
+        assert_eq!(errors.len(), 1);
+        assert_eq!(tokens.last().unwrap().kind, SyntaxKind::True);
+    }
+
     #[test]
     fn invalid_number_reports_error() {
         let (_, errors) = tokenize("1.2.3");