bug: catch invalid assignments and throw a better error message

[kmr-srbh]

'a' = 1

(base) saurabh-kumar@Awadh:~/Projects/System/lpython$ lpython ./examples/example.py
semantic error: Unsupported type in check_is_assign_to_input_param()
 --> ./examples/example.py:1:1
  |
1 | 'a' = 1
  | ^^^ 


Note: Please report unclear or confusing messages as bugs at
https://github.com/lcompilers/lpython/issues.

I think the invalid assignment above should have been caught earlier and reported with a better error message.