[plugin system] Add a utility function (setFontMetrics) to extend builtin fontMetrics (#1269)

* Add and expose addFontMetrics function

* fix typo

* recreate package.json-lock

* Add setFontMetrics function to change the defualt metricMap - change getFontMetrics to getGlobalMetrics

* use new setFontMetrics on main KaTeX object

* fix package-lock.json error by rebuilding it

* Add appropriate tests

* update the snapshot

test/katex-spec.js

@@ -2,6 +2,7 @@
 /* global expect: false */
 /* global it: false */
 /* global describe: false */
+/* global beforeAll: false */
 
 import buildMathML from "../src/buildMathML";
 import buildTree from "../src/buildTree";
@@ -3191,3 +3192,39 @@ describe("Internal __* interface", function() {
         expect(tree.toMarkup()).toEqual(renderedSansMathML);
     });
 });
+
+describe("Extending katex by new fonts and symbols", function() {
+    beforeAll(() => {
+        const fontName = "mockEasternArabicFont";
+        // add eastern arabic numbers to symbols table
+        // these symbols are ۰۱۲۳۴۵۶۷۸۹ and ٠١٢٣٤٥٦٧٨٩
+        for (let number = 0; number <= 9; number++) {
+            const persianNum = String.fromCharCode(0x0660 + number);
+            katex.__defineSymbol(
+                "math", fontName, "textord", persianNum, persianNum);
+            const arabicNum = String.fromCharCode(0x06F0 + number);
+            katex.__defineSymbol(
+                "math", fontName, "textord", arabicNum, arabicNum);
+        }
+    });
+    it("should throw on rendering new symbols with no font metrics", () => {
+        // Lets parse 99^11 in eastern arabic
+        const errorMessage = "Font metrics not found for font: mockEasternArabicFont-Regular.";
+        expect(() => {
+            katex.__renderToDomTree("۹۹^{۱۱}", strictSettings);
+        }).toThrow(errorMessage);
+    });
+    it("should add font metrics to metrics map and render successfully", () => {
+        const mockMetrics = {};
+        // mock font metrics for the symbols that we added previously
+        for (let number = 0; number <= 9; number++) {
+            mockMetrics[0x0660 + number] = [-0.00244140625, 0.6875, 0, 0];
+            mockMetrics[0x06F0 + number] = [-0.00244140625, 0.6875, 0, 0];
+        }
+        katex.__setFontMetrics('mockEasternArabicFont-Regular', mockMetrics);
+        expect("۹۹^{۱۱}").toBuild();
+    });
+    it("Add new font class to new extended symbols", () => {
+        expect(katex.renderToString("۹۹^{۱۱}")).toMatchSnapshot();
+    });
+});