Test emitting let instead of var.

Fixes #856

Author: cristianoc

CHANGELOG.md

@@ -29,6 +29,7 @@
 - Remove redundant space for empty return in generated js code. https://github.com/rescript-lang/rescript-compiler/pull/6745
 - Remove redundant space for export in generated js code. https://github.com/rescript-lang/rescript-compiler/pull/6560
 - Remove empty export blocks in generated js code. https://github.com/rescript-lang/rescript-compiler/pull/6744
+- In generated code, use `let` instead of `var`.
 
 # 11.1.0
 

jscomp/build_tests/custom_namespace/src/demo.bs.js

@@ -1,7 +1,7 @@
 'use strict';
 
 
-var v = 42;
+let v = 42;
 
 exports.v = v;
 /* No side effect */