zsh-completions: Install in standard completion path
zsh-completions: Install in standard completion path

Install zsh-completions in standard zsh_completion path. This should
also avoid unnecessary `$fpath` customization for the user.

Additionally, the test has been updated to validate correctness of
installation without any rc customization.

Also, add `license` attribute.

Closes #76425.

Signed-off-by: Carlo Cabrera <3ffc397d0e4bded29cb84b56167de54c01e3a55b@users.noreply.github.com>
Signed-off-by: BrewTestBot <8a898ee6867e4f2028e63d2a6319b2224641c06c@users.noreply.github.com>

nbari/homebrew-core,lemzwerg/homebrew-core,chrisfinazzo/homebrew-core,adam-moss/homebrew-core,mbcoguno/homebrew-core,lemzwerg/homebrew-core,JCount/homebrew-core,nandub/homebrew-core,Moisan/homebrew-core,mbcoguno/homebrew-core,chrisfinazzo/homebrew-core,Homebrew/homebrew-core,Linuxbrew/homebrew-core,mfikes/homebrew-core,filcab/homebrew-core,Homebrew/homebrew-core,adam-moss/homebrew-core,stgraber/homebrew-core,uyjulian/homebrew-core,nandub/homebrew-core,filcab/homebrew-core,cblecker/homebrew-core,cblecker/homebrew-core,sjackman/homebrew-core,stgraber/homebrew-core,uyjulian/homebrew-core,zyedidia/homebrew-core,Linuxbrew/homebrew-core,sjackman/homebrew-core,zyedidia/homebrew-core,makigumo/homebrew-core,mfikes/homebrew-core,Moisan/homebrew-core,nbari/homebrew-core,mbcoguno/homebrew-core,makigumo/homebrew-core,JCount/homebrew-core