handle backtick in selector

src/selector.rs

@@ -334,6 +334,7 @@ impl<'a> SelectorParser<'a> {
                 c if c.is_control() => {
                     return Err("expected selector.".into());
                 }
+                '`' => return Err("expected selector.".into()),
                 _ => todo!("unimplemented selector"),
             };
         }

tests/error.rs

@@ -52,3 +52,4 @@ error!(
     control_character_starts_selector_inner,
     "a{l {color: foo;}}", "Error: expected selector."
 );
+error!(backtick_in_selector, "a`{}", "Error: expected selector.");