refactor(api): make freeing of return-value opt-in instead of opt out

As only a few API functions make use of explicit freeing of the return
value, make it opt-in instead. The arena is always present under the
hood, so `Arena *arena` arg now doesn't mean anything other than getting
access to this arena. Also it is in principle possible to return an
allocated value while still using the arena as scratch space for other
stuff (unlikely, but there no reason to not allow it).

src/nvim/api/vimscript.c

@@ -51,7 +51,7 @@
 /// @return Dictionary containing information about execution, with these keys:
 ///       - output: (string|nil) Output if `opts.output` is true.
 Dictionary nvim_exec2(uint64_t channel_id, String src, Dict(exec_opts) *opts, Error *err)
-  FUNC_API_SINCE(11)
+  FUNC_API_SINCE(11) FUNC_API_RET_ALLOC
 {
   Dictionary result = ARRAY_DICT_INIT;