Fixed icons alignment of various rendered width

[ehwan]

For #1054. Fixed icons to be right aligned.

Before

After

I'm not sure if it is OK to force right-align icons, but I think it is the simplest solution.

• This follows the same logic as aligning 'Block::Size' or 'Block::SizeValue'.
• However, for --tree, icons are aligned within same level of entries. If all entries in a directory have icons of width 1, no padding will be added even if there are icons with longer widths outside.

---

TODO

• [x] Use cargo fmt
• [x] Add necessary tests
• [x] Update default config/theme in README (if applicable)
• [x] Update man page at lsd/doc/lsd.md (if applicable)

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