Peek current fold structure

[kevinhwang91]

Feature description

To fuck long nested code base.
Sample:

Describe the solution you'd like

Prototype:

          +----------------------------+
tab ->    | level1 | level2 | level3_0*|
          +----------------------------+
          |content3_0                  |
          |    baz                     |
          |    ......                  |
          +----------------------------+

          +----------------------------+
tab ->    | level1 | level2*| level3_0 |
          +----------------------------+
          |content2                    |
          |    bar                     |
          |    ......                  |
          |    level3_0 is folded      |
          +----------------------------+

          +----------------------------+
tab ->    | level1*| level2 | level3_0 |
          +----------------------------+
          |content1                    |
          |    foo                     |
          |    ......                  |
          |    level2 is folded        |
          +----------------------------+

Additional context

No response