Small possible improvements to make parsing easier

[pieterbos]

I made some small adaptations to your parser to make parsing just that little bit easier. This does not improve the functionality of the parser, only the usability.

See https://github.com/nedap/archie/commit/0109786d8110addb562c2bf049c0475041e31efc

• you changed some Lexer tokens to strings in the metadata section. This means i now have to match the actual string in java-code to match this. Changed it to named parser rules so i can use those
• added a SEMICOLON lexer token, So that can be used in the parsing of terminology code constraints with assumed values. Could also be solved (perhaps better?) with a different parser rule
• c_string now directly has a STRING instead of a string_value. Same content, but not consistent with the rest of the grammar and therefore can require some extra code.

[pieterbos]

I still need the SEMICOLON token, but that's really really minor, so lets close this.