js: Take the script file as a command-line argument

Now that we have the beginnings of a parser, let's take the script to
run as a command-line argument and move all the test scripts into
/home/anon/js :^)

To run a script, simply use "js":

$ js my-script.js

To get an AST dump before execution, you can use "js -A"

Base/home/anon/js/simple-scopes.js

@@ -0,0 +1,9 @@
+function foo() {
+    function bar() {
+        var y = 6;
+    }
+    
+    bar()
+    return y;
+}
+foo(); //I should return `undefined` because y is bound to the inner-most enclosing function, i.e the nested one (bar()), therefore, it's undefined in the scope of foo()