fix(rust): don't overwrite vim.g.rustaceanvim if it is defined (#2720)

lua/lazyvim/plugins/extras/lang/rust.lua

@@ -81,7 +81,7 @@ return {
       },
     },
     config = function(_, opts)
-      vim.g.rustaceanvim = vim.tbl_deep_extend("force", {}, opts or {})
+      vim.g.rustaceanvim = vim.tbl_deep_extend("keep", vim.g.rustaceanvim or {}, opts or {})
     end,
   },