Adding some more icons for filenames and extensions (#929)

- Found a better (larger and clearer) Rust icon, so I changed that
(`\u{e68b}`)
- Set emacs config files (`config.el`, `packages.el` etc.) to use the
actual Emacs icon instead of GNU head for Emacs files (`\u{e632}`)
- Perl language extensions (`.pl`, `.pm`, `.pod`, `.plx`) now use the
camel instead of the... circle (`\u{e67e}`)
- Added OCAML extensions (`.ml`, `.mli`, `.mll`)
- Added many Go and Gradle filenames
- Added separate key icons for public keys (extensions) and private keys
(filenames)
- Added a kitty cat for Yarn configuration files
- Switched all the Bash, Zsh, and Csh filenames to use a shebang icon,
instead of the command prompt

[For reference](https://www.nerdfonts.com/cheat-sheet)