fix: make mode() return correct value in ex mode

When the user is in ex mode, a call to mode(1) is documented to return
"cv".  However, it does not currently do so, because the check which
checks for ex mode is nested inside a conditional which is never reached
in ex mode.  Vim uses an explicit check for exmode_active, so let's do
the same thing here.  Add some tests for this case both with a TTY and
in silent mode.

src/nvim/state.c

@@ -180,7 +180,7 @@ char *get_mode(void)
         buf[1] = 'x';
       }
     }
-  } else if (State & CMDLINE) {
+  } else if ((State & CMDLINE) || exmode_active) {
     buf[0] = 'c';
     if (exmode_active) {
       buf[1] = 'v';