fix(ex_normal): spam \n in Ex mode only if in Cmdline mode (#17977)

When using :normal in Ex mode, the editor is no longer in Cmdline mode,
but the exmode_active flag is still set, causing the wrong character to
be spammed in Insert mode, leading to a hang.

test/functional/ex_cmds/normal_spec.lua

@@ -0,0 +1,13 @@
+local helpers = require('test.functional.helpers')(after_each)
+local clear = helpers.clear
+local feed = helpers.feed
+local expect = helpers.expect
+
+before_each(clear)
+
+describe(':normal', function()
+  it('can get out of Insert mode if called from Ex mode #17924', function()
+    feed('gQnormal! Ifoo<CR>')
+    expect('foo')
+  end)
+end)