[lexer.test] Add a test for checking the current number value.

Should fail (not implemented).
[lexer] Store the current float number.

* Static attribute, NaN (not a number) be default.
* Updated the tasks list.
[lexer.test] Check the current word for Identifier of Unknown token.

Should fail (not implemented yet).
[lexer] Store the current word as a static attributer.

* Empty string by default.
* Update the tasks list.