Add new features and modify final fields

[RobEin]

Added method:
getMode()

Added read-only property:
modeStack

Modified fields:
DEFAULT_MODE
MORE
SKIP
DEFAULT_TOKEN_CHANNEL
HIDDEN
MIN_CHAR_VALUE
MAX_CHAR_VALUE

Final fields cannot be used in the case clause of the switch statement.
error message:

"The expression of a constant pattern must be a valid constant."

e.g:
The folowing example does not work with final field but it works with constant field:

switch (curMode) {
  case Lexer.DEFAULT_MODE:
    ...
}

It would be worthwhile to review the other classes in this regard.