Replace the current Zsh completion with a Zsh completion solution based
on Go completions. This allows to support custom completions (based
on Go completions), but also to standardize the behavior of completion
across all shells.
Also, add support to Go completions for the bash completion annotations:
BashCompFilenameExt (including Command.MarkFlagFilename() family)
- still supported by zsh
BashCompSubdirsInDir - now supported by zsh
BashCompOneRequiredFlag (including Command.MarkFlagRequired() family)
- now supported by zsh and fish
Finally, remove the suggestin of the = form of flag completion.
The = form is supported, but it will not be suggested to avoid having
duplicated suggestions.
