fix(parser): correct destructor type spacing to match declarations

The previous commit added %destructor directives but used inconsistent
spacing in nested template types. The %type declarations used spaces
between closing angle brackets (> > > >) but %destructor used (>>>>),
causing Bison to not associate the destructors with any symbols.

Modern Bison 3.8.2 detected this with warnings:
  "type <...> is used, but is not associated to any symbol"

This meant vector type destructors were silently ignored, leaving those
memory leaks from issue #3448 unfixed. Fixed by matching the spacing
exactly between %type and %destructor declarations.

Also enhanced comments to explain why destructor bodies are empty when
using api.value.type variant.

Author: fzipi