feat(ui): "append" parameter for "msg_show" UI events

Problem:  Consecutive "msg_show" events stemming from an `:echon`
          command are supposed to be appended without a newline, this
          information is not encoded in the "msg_show" event.
Solution: Add an "append" parameter to the "msg_show" event that is set
          to true to indicate the message should not start on a new line.
Considered alternative: Emit a newline for the common case instead at the
start of a new message. That way UIs can more closely follow the logic
as it is implemented for the message grid currently. This would be a
breaking change. The "append" parameter seems OK.

test/functional/lua/ui_event_spec.lua

@@ -221,7 +221,6 @@ describe('vim.ui_attach', function()
       messages = {
         {
           content = { { '\nSave changes?\n', 6, 10 } },
-          history = false,
           kind = 'confirm',
         },
       },
@@ -477,7 +476,6 @@ describe('vim.ui_attach', function()
         },
         {
           content = { { 'Press ENTER or type command to continue', 100, 18 } },
-          history = false,
           kind = 'return_prompt',
         },
       },
@@ -530,7 +528,6 @@ describe('vim.ui_attach', function()
         },
         {
           content = { { 'Press ENTER or type command to continue', 100, 18 } },
-          history = false,
           kind = 'return_prompt',
         },
       },