Provide an option to require explicit `var` declarations

[christoph_schlumpf]

One more reason to provide an option to require explicit var declarations:

@AlisterBurt

… the following compiles and runs without issue:

comptime counter: Int = 1


def main():
    # compiler prevents modification of compile time constants... or does it?
    print(counter)
    counter = counter + 3
    print(counter)

(mojo-by-example) ➜ mojo 03_constants_variables.mojo
1
4

Inside the function counter is implicitly declared as a new local variable shadowing the global constant… tricky!