Extend parser to accept '_' in numbers. * NEWS: Announce change. * lex.ll: Define NUMBER to be a NUMREAL (real number) or NUMHEX (hex number). Define NUMREAL to begin with a digit (D) followed by more digits or an '_'. Define NUMHEX to begin with 0[xX], a hex digit (a-fA-F0-9), followed by '_' or more hex digits. Define EXPON to have a digit (D) followed by more digits or an '_'. * lex.ll (octave_base_lexer::handle_number): Strip out any underscores before processing number with sscanf. * parser.tst: Add tests for new behavior.

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