Update js-mode "for" to keep up with modern JS syntax (AndreaCrotti#514)

"let" contains the scope of the variable to the loop, while "var" always
declares global scope.

Following up AndreaCrotti#305

Author: xuhdev

snippets/js-mode/for

@@ -1,6 +1,6 @@
 # -*- mode: snippet; require-final-newline: nil -*-
 # name: for
 # --
-for (var ${1:i} = ${2:0}; $1 < ${3:collection}.length; $1++) {
+for (let ${1:i} = ${2:0}; $1 < ${3:collection}.length; $1++) {
   $0
 }