Enable the `no-var` linting rule for `src/core/pattern.js`
Enable the `no-var` linting rule for `src/core/pattern.js`

This is mostly done using `gulp lint --fix` with a few manual changes in
the following diff:

```diff
diff --git a/src/core/pattern.js b/src/core/pattern.js
index 365491ed3..eedd8b686 100644
--- a/src/core/pattern.js
+++ b/src/core/pattern.js
@@ -105,7 +105,7 @@ const Pattern = (function PatternClosure() {
   return Pattern;
 })();

-var Shadings = {};
+const Shadings = {};

 // A small number to offset the first/last color stops so we can insert ones to
 // support extend. Number.MIN_VALUE is too small and breaks the extend.
@@ -597,16 +597,15 @@ Shadings.Mesh = (function MeshClosure() {
       if (!(0 <= f && f <= 3)) {
         throw new FormatError("Unknown type6 flag");
       }
-      var i, ii;
       const pi = coords.length;
-      for (i = 0, ii = f !== 0 ? 8 : 12; i < ii; i++) {
+      for (let i = 0, ii = f !== 0 ? 8 : 12; i < ii; i++) {
         coords.push(reader.readCoordinate());
       }
       const ci = colors.length;
-      for (i = 0, ii = f !== 0 ? 2 : 4; i < ii; i++) {
+      for (let i = 0, ii = f !== 0 ? 2 : 4; i < ii; i++) {
         colors.push(reader.readComponents());
       }
-      var tmp1, tmp2, tmp3, tmp4;
+      let tmp1, tmp2, tmp3, tmp4;
       switch (f) {
         // prettier-ignore
         case 0:
@@ -729,16 +728,15 @@ Shadings.Mesh = (function MeshClosure() {
       if (!(0 <= f && f <= 3)) {
         throw new FormatError("Unknown type7 flag");
       }
-      var i, ii;
       const pi = coords.length;
-      for (i = 0, ii = f !== 0 ? 12 : 16; i < ii; i++) {
+      for (let i = 0, ii = f !== 0 ? 12 : 16; i < ii; i++) {
         coords.push(reader.readCoordinate());
       }
       const ci = colors.length;
-      for (i = 0, ii = f !== 0 ? 2 : 4; i < ii; i++) {
+      for (let i = 0, ii = f !== 0 ? 2 : 4; i < ii; i++) {
         colors.push(reader.readComponents());
       }
-      var tmp1, tmp2, tmp3, tmp4;
+      let tmp1, tmp2, tmp3, tmp4;
       switch (f) {
         // prettier-ignore
         case 0:
@@ -897,7 +895,7 @@ Shadings.Mesh = (function MeshClosure() {
         decodeType4Shading(this, reader);
         break;
       case ShadingType.LATTICE_FORM_MESH:
-        var verticesPerRow = dict.get("VerticesPerRow") | 0;
+        const verticesPerRow = dict.get("VerticesPerRow") | 0;
         if (verticesPerRow < 2) {
           throw new FormatError("Invalid VerticesPerRow");
         }
```

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