fix: parse IMPLEMENTS keyword after EXTENDS

[mhasel]

Moves the parse-interface logic after the parsing of EXTENDS keywords in parse_pou.
This allows correct parsing of e.g. FUNCTION_BLOCK bar EXTENDS foo IMPLEMENTS baz, registering the implemented interface for the derived function block.

[volsa]

Looks good, the error message is a bit weird when you have something like IMPLEMENTS <...> EXTENDS <...> but that is generally something we have to improve on the parser side