[str_utils] add a function to iterate over a stream and get the next word.

* Add a related test.
* Also add two other functions to test if the word is a comment and a function
  to skip the current line.
[lexer] Use the 'next_word' function in the lexer class.

* Rename 'next_valid_word' to 'next_word'.
* Also updated some test cases.
[test] str utils: skipline and next_word passed.
[lexer] Take into account commented line in parsing.

Updated the tasks list.