Objective-C is an object-oriented extractor that already emits @interface,
@implementation and @protocol type nodes, but plain `enum` and
`typedef enum` definitions were dropped entirely: both the enum type and all
of its enumerators were invisible in the graph.
Handle `enum_specifier` (and the `typedef enum { ... } Name;` form, where the
name is the typedef alias), emitting a type node plus a node and `case_of` edge
per enumerator — the same shape every other language with enums uses (Java
#1719, C++ #3939, Swift). A nameless enum and a bodyless `enum X y;` reference
are skipped so no phantom node is minted.
NS_ENUM/NS_OPTIONS is deliberately out of scope: it is a macro call that
tree-sitter-objc parses into an ERROR subtree, so there is no clean AST to
read. Documented in the test.
Add regression tests for the plain, typedef, and skip cases.
