Add icon for .cmd

[spenserblack]

.cmd files are almost identical to .bat, with slightly different syntax and error handling.

---

TODO

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

[spenserblack]

Regarding the "TODO", I naively grepped for .bat and didn't find any tests or docs (just the one line in src/theme/icon.rs), so I'm assuming the other steps aren't necessary for this change.