api/lua: make nvim_execute_lua use native lua floats, not special tables

Rationale: the purpose of nvim_execute_lua is to simply call lua code with lua
values. If a lua function expects a floating point value, it should be enough
to specify a float as argument to nvim_execute_lua.

However, make sure to preserve the existing roundtripping behavior of
API values when using `vim.api` functions. This is covered by existing
lua/api_spec.lua tests.

src/nvim/generators/gen_api_dispatch.lua

@@ -441,7 +441,7 @@ local function process_function(fn)
     end
     write_shifted_output(output, string.format([[
     const %s ret = %s(%s);
-    nlua_push_%s(lstate, ret);
+    nlua_push_%s(lstate, ret, true);
     api_free_%s(ret);
   %s
   %s