Speed up insertion and deletion of values in SortedList by batching and deferring to idle task

[Tyriar]

Fixes #4911

Test setup:

• 10000 scrollback filled
• Decoration stress test button
• ~10000 * 20 = ~200,000 decorations

Before:

Insert ~4s
Delete ~4s

After:

Insert ~330ms
Delete ~93ms

There's still a decent amount of GC happening on insert, but a lot of it has to happen due to the variables being used in callbacks. It's reasonable when you consider it's 200000 decorations.