fix: use zsh idioms and quote variables in shell config

- Use [[ ]] instead of [ ] for conditionals (zsh idiom)
- Quote variable expansions in ZINIT_HOME checks and dirname call
- Quote dollar-1 in mkcd function
- Remove extra blank line in .vimrc

Nightshift-Task: lint-fix
Nightshift-Ref: https://github.com/marcus/nightshift

.vimrc

@@ -24,7 +24,6 @@ set backupdir=~/.vim/backup//
 set nobackup
 set mouse=a             " mouse support
 
-
 noremap <Up> <NOP>
 noremap <Down> <NOP>
 noremap <Left> <NOP>