[js] Enable prefer-const linting rule

Manually convert some lets into consts

.eslintrc.js

@@ -13,6 +13,7 @@ module.exports = {
       "prefer-rest-params": "off",
       "prefer-spread": "off",
       "no-invalid-this": "off",
-      "no-throw-literal": "off"
+      "no-throw-literal": "off",
+      "prefer-const": "error"
     }
 };

src/vm/js/nqp-runtime/deserialization.js

@@ -194,15 +194,14 @@ class BinaryCursor {
   }
 
   locateThing(thingType) {
-    let packed;
     let scId;
     let index;
 
     const PACKED_SC_SHIFT = 20;
     const PACKED_SC_OVERFLOW = 0xfff;
     const PACKED_SC_IDX_MASK = 0x000fffff;
 
-    packed = this.varint();
+    const packed = this.varint();
     scId = packed >>> PACKED_SC_SHIFT;
     index = packed & PACKED_SC_IDX_MASK;
 
@@ -255,17 +254,15 @@ class BinaryCursor {
   // XXX TODO - length checks
   varint() {
     let result;
-    let first;
-    let need;
     const buffer = this.buffer;
 
-    first = buffer.readUInt8(this.offset++);
+    const first = buffer.readUInt8(this.offset++);
 
     if (first & 0x80) {
       return first - 129;
     }
 
-    need = first >> 4;
+    const need = first >> 4;
 
     if (!need) {
       // unrolled loop for optimization

src/vm/js/nqp-runtime/null.js

@@ -1,6 +1,6 @@
 'use strict';
 const NQPObject = require('./nqp-object.js');
-let singleton;
+let singleton; // eslint-disable-line prefer-const
 class Null extends NQPObject {
   $$toBool(ctx) {
     return 0;