feat(events)!: support Optionset modified, drop BufModifiedSet #35610

Problem:
BufModifiedSet autocmd only triggered for current buffer during
redraw, causing delayed events when :wa writes non-current buffers.

Solution:
- Use the aucmd_defer approach to implement `Optionset modified`.
- Drop BufModifiedSet.

runtime/lua/vim/_meta/api_keysets.gen.lua

@@ -88,7 +88,6 @@ error('Cannot require a meta file')
 --- |'BufFilePre'
 --- |'BufHidden'
 --- |'BufLeave'
---- |'BufModifiedSet'
 --- |'BufNew'
 --- |'BufNewFile'
 --- |'BufRead'