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/api_keysets.lua

@@ -160,6 +160,7 @@ error('Cannot require a meta file')
 --- |'LspProgress'
 --- |'LspRequest'
 --- |'LspTokenUpdate'
+--- |'MarkSet'
 --- |'MenuPopup'
 --- |'ModeChanged'
 --- |'OptionSet'