feat(messages): "g<" mapping for ext_messages

Problem:  Cannot use "g<" mapping with ext_messages. Mapping displays
          the scrollback buffer since the last command, but the
          scrollback buffer is not populated with ext_messages.
Solution: With ext_messages; store messages in the history that otherwise
          wouldn't be. Mark them as temporary messages to be deleted when
          the scrollback buffer would be cleared. To this end, make the
          message history a doubly-linked list such that messages can be
          removed from an arbitrary position.
Outlook:  Default ext_messages UI might not show the hit-enter prompt
          so we want "g<" to work as a recommended way to show messages
          for the last command (prompted by an indicator).

test/functional/ui/messages_spec.lua

@@ -1571,6 +1571,48 @@ stack traceback:
       end,
     })
   end)
+
+  it('g< mapping shows recent messages', function()
+    command('echo "foo" | echo "bar"')
+    local s1 = [[
+      ^                         |
+      {1:~                        }|*4
+    ]]
+    screen:expect({
+      grid = s1,
+      messages = {
+        {
+          content = { { 'bar' } },
+          history = false,
+          kind = 'echo',
+        },
+      },
+    })
+    feed(':messages<CR>g<lt>')
+    screen:expect({
+      grid = [[
+        ^                         |
+        {1:~                        }|*4
+      ]],
+      messages = {
+        {
+          content = { { 'Press ENTER or type command to continue', 6, 18 } },
+          history = false,
+          kind = 'return_prompt',
+        },
+      },
+      msg_history = {
+        {
+          content = { { 'foo' } },
+          kind = 'echo',
+        },
+        {
+          content = { { 'bar' } },
+          kind = 'echo',
+        },
+      },
+    })
+  end)
 end)
 
 describe('ui/builtin messages', function()