parser: accept group inside oneof.

[VeaaC]

group is legal as a oneof member (spec: OneofGroupDecl), but rapidprotoc rejected the
whole schema with unexpected input. A oneof is not a scope, so parse_oneof now returns an
OneofBundle — mirroring the existing ExtendBundle — and the synthesized message hoists to
the enclosing message while the delimited field joins the oneof. parse_oneof moved into
parser.cpp's anonymous namespace alongside parse_extend, which was never public.

Aligning the two element lists it touches against
the grammar removed two stray acceptances:

• ; between oneof members. MessageElement lists EmptyDecl; OneofElement does not.
• ; or an option in an extend body. ExtensionElement is the only element list
  carrying neither. That is deliberate: ExtensionFieldDeclIdentifier subtracts only the
  cardinality keywords, not option — so option is a legal extension-field type name
  there, and the OptionDecl branch was misparsing extend M { option x = 100; } as an
  option declaration. ExtendNode::options is removed with it; the only consumer was a
  resolve_extend call that could never fire.

A labelled oneof member is refused with protoc's own wording, at protoc's exact column
(previously unexpected input, pointing past the label). oneof p { optional x = 1; } in a
file declaring a message named optional is now correctly rejected rather than parsed as a
field of that type. An empty oneof body stays accepted: the grammar admits it, and the
prose rule requiring one member is a semantic check we don't make.