Enable the ESLint `no-var` rule in the `src/core/font_renderer.js` file
Enable the ESLint `no-var` rule in the `src/core/font_renderer.js` file

Note that the majority of these changes were done automatically, by using `gulp lint --fix`, and the manual changes were limited to the following diff:

```diff
diff --git a/src/core/font_renderer.js b/src/core/font_renderer.js
index e1538c481..00f5424cd 100644
--- a/src/core/font_renderer.js
+++ b/src/core/font_renderer.js
@@ -152,9 +152,9 @@ const FontRendererFactory = (function FontRendererFactoryClosure() {
   }

   function lookupCmap(ranges, unicode) {
-    let code = unicode.codePointAt(0),
-      gid = 0;
-    let l = 0,
+    const code = unicode.codePointAt(0);
+    let gid = 0,
+      l = 0,
       r = ranges.length - 1;
     while (l < r) {
       const c = (l + r + 1) >> 1;
@@ -199,7 +199,7 @@ const FontRendererFactory = (function FontRendererFactoryClosure() {
         flags = (code[i] << 8) | code[i + 1];
         const glyphIndex = (code[i + 2] << 8) | code[i + 3];
         i += 4;
-        var arg1, arg2;
+        let arg1, arg2;
         if (flags & 0x01) {
           arg1 = ((code[i] << 24) | (code[i + 1] << 16)) >> 16;
           arg2 = ((code[i + 2] << 24) | (code[i + 3] << 16)) >> 16;
@@ -366,7 +366,7 @@ const FontRendererFactory = (function FontRendererFactoryClosure() {
       while (i < code.length) {
         let stackClean = false;
         let v = code[i++];
-        var xa, xb, ya, yb, y1, y2, y3, n, subrCode;
+        let xa, xb, ya, yb, y1, y2, y3, n, subrCode;
         switch (v) {
           case 1: // hstem
             stems += stack.length >> 1;
@@ -494,7 +494,7 @@ const FontRendererFactory = (function FontRendererFactoryClosure() {
                 bezierCurveTo(xa, y2, xb, y3, x, y);
                 break;
               case 37: // flex1
-                var x0 = x,
+                const x0 = x,
                   y0 = y;
                 xa = x + stack.shift();
                 ya = y + stack.shift();
```

Snuffleupagus/pdf.js,timvandermeij/pdf.js,macroplant/pdf.js,timvandermeij/pdf.js,mozilla/pdf.js,Snuffleupagus/pdf.js,nawawi/pdf.js,macroplant/pdf.js,Shoobx/pdf.js,nawawi/pdf.js,showpad/mozilla-pdf.js,timvandermeij/pdf.js,Snuffleupagus/pdf.js,xavier114fch/pdf.js,mozilla/pdf.js,Shoobx/pdf.js,Shoobx/pdf.js,nawawi/pdf.js,macroplant/pdf.js,xavier114fch/pdf.js,mozilla/pdf.js,showpad/mozilla-pdf.js,showpad/mozilla-pdf.js