fix(editor): respect [+cmd] when executing :drop #33339

Problem:
Normally, `:drop +41 foo.txt` will open foo.txt with the cursor on line
41. But if foo.txt is already open, it instead is a no-op, even if the
cursor is on a different line.

Steps to reproduce:

    nvim --clean foo.txt
    :drop +30 foo.txt

Solution:
Handle +cmd in ex_drop().

src/nvim/ex_cmds2.c

@@ -888,6 +888,17 @@ void ex_drop(exarg_T *eap)
       if (curbuf->b_ml.ml_flags & ML_EMPTY) {
         ex_rewind(eap);
       }
+
+      // execute [+cmd]
+      if (eap->do_ecmd_cmd) {
+        bool did_set_swapcommand = set_swapcommand(eap->do_ecmd_cmd, 0);
+        do_cmdline(eap->do_ecmd_cmd, NULL, NULL, DOCMD_VERBOSE);
+        if (did_set_swapcommand) {
+          set_vim_var_string(VV_SWAPCOMMAND, NULL, -1);
+        }
+      }
+
+      // no need to execute [++opts] - they only apply for newly loaded buffers.
       return;
     }
   }