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/generators/nvim_version.lua.in

@@ -0,0 +1,9 @@
+return {
+  {"major", ${NVIM_VERSION_MAJOR}},
+  {"minor", ${NVIM_VERSION_MINOR}},
+  {"patch", ${NVIM_VERSION_PATCH}},
+  {"prerelease", "$NVIM_VERSION_PRERELEASE" ~= ""},
+  {"api_level", ${NVIM_API_LEVEL}},
+  {"api_compatible", ${NVIM_API_LEVEL_COMPAT}},
+  {"api_prerelease", ${NVIM_API_PRERELEASE}},
+}