Fixing lexer issues

[Larsouille25]

All of these issues are in the same issue because it's easier to follow and because there are only small errors.

• [x] When a non-alphanumeric char is in an ident, (e.g: a@), the error is reported an char after:

  Err: Lexer, in file `<stdin>` at line 1 :
  ... |
  1  | a@
  ... |   ^
  Illegal Character

• [x] Remove the magic if (while fixing the error) in the lexer, line 85
• [x] When an underscore is in a identifier, an error occured but not the correct one,

  ~> func foo_bar(a, b)
  .eof
  cannot parse integer from empty string

• [x] Make it possible to use _ in identifiers, at the beginning and in the middle.

[Larsouille25]

It seems like the error with the IllegalCharError doesn't point to the correct char is even worse now

[Larsouille25]

It seems like, now the error was been fixed, so I close this issue.