refactor(metadata): generate all metadata in lua

Then we can just load metadata in C as a single msgpack blob. Which also
can be used directly as binarly data, instead of first unpacking all the
functions and ui_events metadata to immediately pack it again, which was
a bit of a silly walk (and one extra usecase of `msgpack_rpc_from_object`
which will get yak shaved in the next PR)

src/nvim/api/ui.h

@@ -7,6 +7,20 @@
 #include "nvim/types_defs.h"  // IWYU pragma: keep
 #include "nvim/ui_defs.h"  // IWYU pragma: keep
 
+/// Keep in sync with UIExtension in ui_defs.h
+EXTERN const char *ui_ext_names[] INIT( = {
+  "ext_cmdline",
+  "ext_popupmenu",
+  "ext_tabline",
+  "ext_wildmenu",
+  "ext_messages",
+  "ext_linegrid",
+  "ext_multigrid",
+  "ext_hlstate",
+  "ext_termcolors",
+  "_debug_float",
+});
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "api/ui.h.generated.h"
 # include "ui_events_remote.h.generated.h"