Support Unicode colon equals (#1151)

* Support Unicode colon equals

This PR adds support for three Unicode characters: `≔` `≕` `⩴`

* Fix error in \Coloneqq

test/katex-spec.js

@@ -2915,7 +2915,7 @@ describe("Unicode", function() {
     });
 
     it("should parse relations", function() {
-        expect("∈∋∝∼∽≂≃≅≈≊≍≎≏≐≑≒≓≖≗≜≡≤≥≦≧≫≬≳≷≺≻≼≽≾≿∴∵∣").toParse();
+        expect("∈∋∝∼∽≂≃≅≈≊≍≎≏≐≑≒≓≖≗≜≡≤≥≦≧≫≬≳≷≺≻≼≽≾≿∴∵∣≔≕⩴").toParse();
     });
 
     it("should parse big operators", function() {