Remark the folded line and export the data

[kevinhwang91]

Feature description

A folded line actually carry an extmark, we can remap i and a to insert new text to add 'notes' when reviewing the code.
And export the extmarks based on foldlevel to make us understand the meaning of the code we remarked forever unless the file is changed.

I like this feature because I am lazy to make notes but I like reading the fucking source code.

Describe the solution you'd like

Support undo/redo first, and make the extmark more stable when we are making notes.

Additional context

No response