feat: add preview functionality to user commands

Adds a Lua-only `preview` flag to user commands which allows the command to be incrementally previewed like `:substitute` when 'inccommand' is set.

src/nvim/api/keysets.lua

@@ -53,6 +53,7 @@ return {
     "force";
     "keepscript";
     "nargs";
+    "preview";
     "range";
     "register";
   };