Support some Unicode Mathematical Alphanumeric Symbols (#1232)

* Support some  Unicode Mathematical Alphanumeric Symbols

This PR adds support for some of the characters in Unicode range U+1D400 to U+1D7FF, [Mathematical Alphanumeric Symbols](https://www.unicode.org/charts/PDF/U1D400.pdf).  Specifically, it adds support for:
* A-Z a-z bold
* A-Z a-z bold italic
* A-Z a-z Fractur
* A-Z a-z sans-serif
* A-Z double struck
•	A-Z script

Addresses issue #1215 and parts of #260.

* Fix lint error

* Fix MathML, and pick up review comments

* Fix lint error

* Add text mode. Remove sans-serif.

* Fix lint error

* Fixed \mathrm, added screenshotter test

* Change screenshotter test to an array

* Add screenshots

* Picked up review comments. Add characters

test/katex-spec.js

@@ -3006,7 +3006,7 @@ describe("Unicode", function() {
         expect("±×÷∓∔∧∨∩∪≀⊎⊓⊔⊕⊖⊗⊘⊙⊚⊛⊝⊞⊟⊠⊡⊺⊻⊼⋇⋉⋊⋋⋌⋎⋏⋒⋓⩞\u22C5").toParse();
     });
 
-    it("should parse delimeters", function() {
+    it("should build delimiters", function() {
         expect("\\left\u230A\\frac{a}{b}\\right\u230B").toBuild();
         expect("\\left\u2308\\frac{a}{b}\\right\u2308").toBuild();
         expect("\\left\u27ee\\frac{a}{b}\\right\u27ef").toBuild();
@@ -3014,6 +3014,41 @@ describe("Unicode", function() {
         expect("\\left\u23b0\\frac{a}{b}\\right\u23b1").toBuild();
         expect("┌x┐ └x┘").toBuild();
     });
+
+    it("should build some surrogate pairs", function() {
+        let wideCharStr = "";
+        wideCharStr += String.fromCharCode(0xD835, 0xDC00);   // bold A
+        wideCharStr += String.fromCharCode(0xD835, 0xDC68);   // bold italic A
+        wideCharStr += String.fromCharCode(0xD835, 0xDD04);   // Fraktur A
+        wideCharStr += String.fromCharCode(0xD835, 0xDD38);   // double-struck
+        wideCharStr += String.fromCharCode(0xD835, 0xDC9C);   // script A
+        wideCharStr += String.fromCharCode(0xD835, 0xDDA0);   // sans serif A
+        wideCharStr += String.fromCharCode(0xD835, 0xDDD4);   // bold sans A
+        wideCharStr += String.fromCharCode(0xD835, 0xDE08);   // italic sans A
+        wideCharStr += String.fromCharCode(0xD835, 0xDE70);   // monospace A
+        wideCharStr += String.fromCharCode(0xD835, 0xDFCE);   // bold zero
+        wideCharStr += String.fromCharCode(0xD835, 0xDFE2);   // sans serif zero
+        wideCharStr += String.fromCharCode(0xD835, 0xDFEC);   // bold sans zero
+        wideCharStr += String.fromCharCode(0xD835, 0xDFF6);   // monospace zero
+        expect(wideCharStr).toBuild();
+
+        let wideCharText = "\text{";
+        wideCharText += String.fromCharCode(0xD835, 0xDC00);   // bold A
+        wideCharText += String.fromCharCode(0xD835, 0xDC68);   // bold italic A
+        wideCharText += String.fromCharCode(0xD835, 0xDD04);   // Fraktur A
+        wideCharText += String.fromCharCode(0xD835, 0xDD38);   // double-struck
+        wideCharText += String.fromCharCode(0xD835, 0xDC9C);   // script A
+        wideCharText += String.fromCharCode(0xD835, 0xDDA0);   // sans serif A
+        wideCharText += String.fromCharCode(0xD835, 0xDDD4);   // bold sans A
+        wideCharText += String.fromCharCode(0xD835, 0xDE08);   // italic sans A
+        wideCharText += String.fromCharCode(0xD835, 0xDE70);   // monospace A
+        wideCharText += String.fromCharCode(0xD835, 0xDFCE);   // bold zero
+        wideCharText += String.fromCharCode(0xD835, 0xDFE2);   // sans serif zero
+        wideCharText += String.fromCharCode(0xD835, 0xDFEC);   // bold sans zero
+        wideCharText += String.fromCharCode(0xD835, 0xDFF6);   // monospace zero
+        wideCharText += "}";
+        expect(wideCharText).toBuild();
+    });
 });
 
 describe("The maxSize setting", function() {