refactor: rename SymbolTable fields and flatten struct to StringBuffer alias

Replace the nested `SymbolTable` struct (containing a `StringBuffer names` field) with a direct `typedef StringBuffer SymbolTable`, updating all internal accesses from `symbols->names.data` to `symbols->data` and `symbols->names.count` to `symbols->count`. Rename `vm->methods` to `vm->methodNames` and `vm->globalSymbols` to `vm->globalNames` across the codebase, including debug printing, compiler symbol resolution, core initialization, and VM interpreter error messages. Remove the stale TODO comment about error codes in `wren.h` and adjust the `instantiate` method name to include a leading space to prevent direct user invocation.

include/wren.h

@@ -85,7 +85,6 @@ WrenVM* wrenNewVM(WrenConfiguration* configuration);
 void wrenFreeVM(WrenVM* vm);
 
 // Runs [source], a string of Wren source code in a new fiber in [vm].
-// TODO: Define error codes.
 WrenInterpretResult wrenInterpret(WrenVM* vm, const char* sourcePath,
                                   const char* source);