feat(ui): specify whether msg_show event is added to history

Pass along whether message in msg_show event is added to the internal
:messages history.

test/functional/lua/ui_event_spec.lua

@@ -265,6 +265,7 @@ describe('vim.ui_attach', function()
       messages = {
         {
           content = { { 'E122: Function Foo already exists, add ! to replace it', 9, 6 } },
+          history = true,
           kind = 'emsg',
         },
       },
@@ -283,6 +284,7 @@ describe('vim.ui_attach', function()
       messages = {
         {
           content = { { 'replace with Replacement (y/n/a/q/l/^E/^Y)?', 6, 18 } },
+          history = true,
           kind = 'confirm_sub',
         },
       },
@@ -300,10 +302,12 @@ describe('vim.ui_attach', function()
       messages = {
         {
           content = { { 'Select:\nOne\nTwo\n' } },
+          history = false,
           kind = 'list_cmd',
         },
         {
           content = { { 'Type number and <Enter> or click with the mouse (q or empty cancels): ' } },
+          history = false,
           kind = 'number_prompt',
         },
       },
@@ -382,6 +386,7 @@ describe('vim.ui_attach', function()
               6,
             },
           },
+          history = true,
           kind = 'lua_error',
         },
         {
@@ -392,10 +397,12 @@ describe('vim.ui_attach', function()
               6,
             },
           },
+          history = true,
           kind = 'lua_error',
         },
         {
           content = { { 'Press ENTER or type command to continue', 100, 18 } },
+          history = false,
           kind = 'return_prompt',
         },
       },