feat: add smart left indent RFC8792 folding

[panva]

Adds a smart indentation variant for automated RFC 8792 folding.

The new smartN fold option can be used in the same position as leftN (they are mutually exclusive), for example:

{::include-fold69hardsmart2dry generated.json}

Unlike leftN, which uses a fixed continuation indentation, smartN computes the continuation indentation from the line being folded:

continuation indent = leading spaces of first folded line + N

This is useful for nested structured data such as JSON, where folded continuation lines should stay visually aligned with the surrounding nesting level.

Since you moved the docs to the wiki I'll leave those to you to update if this gets accepted.

---

Example input:

{
  "outer": {
    "array": [
      {
        "description": "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
        "nested": {
          "token": "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb"
        }
      }
    ]
  }
}

With fold69hardsmart2dry:

NOTE: '\' line wrapping per RFC 8792

{
  "outer": {
    "array": [
      {
        "description": "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\
          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa\
          aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa",
        "nested": {
          "token": "bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb\
            bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb\
            bbbbbbbbbbbbbbbb"
        }
      }
    ]
  }
}

[cabo]

Nice!

Sorry for nitpicking on the code.

[panva]

Sorry for nitpicking on the code.

No worries, I welcome the direction.

[cabo]

Some more thoughts...

[cabo]

Hmm, makes me wonder whether there should be a smart_right (right-aligned, but not using more of the line to the left than smart indentation).

[panva]

possible in a follow up, and possibly after there's a test suite, this is getting to a point where changes could use a set of vectors to test with.