fix(startup): set full_screen when in ex_mode

Problem Description: In ex_mode, the default_grid.chars are not allocated, and subsequently,
the w_grid.target in curwin is not allocated to default_grid in update_screen. This leads to
a null pointer crash when the completion function is executed in ex_mode.

Solution: Set full_screen when in ex_mode to ensure that default_grid is allocated.

test/functional/core/main_spec.lua

@@ -73,6 +73,18 @@ describe('command-line option', function()
       eq(#'100500\n', attrs.size)
     end)
 
+    it('does not crash when run completion in ex mode', function()
+      fn.system({
+        nvim_prog_abs(),
+        '--clean',
+        '-e',
+        '-s',
+        '--cmd',
+        'exe "norm! i\\<C-X>\\<C-V>"',
+      })
+      eq(0, eval('v:shell_error'))
+    end)
+
     it('does not crash after reading from stdin in non-headless mode', function()
       skip(is_os('win'))
       local screen = Screen.new(40, 8)