Fix recursion output indentation and spacing

[Shonas301]

I've been using lsd -R for deep walks and it's hard to follow — entries sit
flush-left regardless of depth, and there are multiple blank lines between
subdirectory blocks. No visual nesting at all.

Changes:

• indent each recursive entry by 2 * depth spaces (both headers and content)
  so the nesting reads visually
• collapse runs of blank lines between directory blocks down to a single separator
• tests covering per-depth indent at depth 1 and 2, plus a no-double-blank invariant

Before:

subdir:
file.rs

subdir/nested:


deep.rs

After:

  subdir:
    file.rs

    subdir/nested:
      deep.rs

Display-layer only — nothing in traversal or meta collection changed.

---

TODO

• Use cargo fmt
• Add necessary tests
• Update README (if applicable)
• Update config sample file in doc/samples (if applicable)
• Update icon sample file in doc/samples (if applicable)
• Update color sample file in doc/samples (if applicable)
• Update man page at lsd/doc/lsd.md (if applicable)

[Shonas301]

Sorry about the cold open of this and #1203 ! screen shot is attached for what it looks like! If there are changes requested just lmk, nbd, if you don't think it belongs in the upstream just lmk.