Fix a #line directive handling

Requirements to run

• JDK 11

Requirements to build

• JDK 17

New

• Add Tokens.isKeywordOrIdentifier(Token)
• Add Tokens.reparseLineDigitsAndFilename()
• Add TokenType.DIGITS

Changed

• Add test cases for the Tokens class

Fixed

• Fix #line directive handling so that the first argument generates a token with type DIGITS and, if a second argument exists, it generates a token with type FILENAME.