Prevent top level variable creation

[BarbourSmith]

Since top level definitions should be immutable we should be blocking users from creating variables using var or let at the top level. Probably just throwing a clear error is the solution.

The example which brought this up is:

let str = '';

for (let i = 0; i < 9; i++) {
  str = str + i;
}

console.log(str);