feat(events): MarkSet event, aucmd_defer() #35793

Problem:
- Can't subscribe to "mark" events.
- Executing events is risky because they can't be deferred.

Solution:
- Introduce `MarkSet` event.
- Introduce `aucmd_defer()`.

Helped-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

runtime/lua/vim/_meta/options.lua

@@ -2213,6 +2213,7 @@ vim.go.ei = vim.go.eventignore
 --- 	`LspProgress`,
 --- 	`LspRequest`,
 --- 	`LspTokenUpdate`,
+--- 	`MarkSet`,
 --- 	`MenuPopup`,
 --- 	`ModeChanged`,
 --- 	`OptionSet`,