Add double-equals homoglyph

tests/ui/parser/unicode-chars.rs

@@ -6,4 +6,7 @@ fn main() {
     //~^ ERROR unknown start of token: \u{a0}
     //~^^ NOTE character appears 3 more times
     //~^^^ HELP Unicode character ' ' (No-Break Space) looks like ' ' (Space), but it is not
+    let _ = 1 ⩵ 2;
+    //~^ ERROR unknown start of token
+    //~^^ HELP Unicode character '⩵' (Two Consecutive Equals Signs) looks like '==' (Double Equals Sign), but it is not
 }