Replace `click-completion` with `click`'s own completion code

[marlonrichert]

Is your feature request related to a problem? Please describe.

Since version 8, click itself provides shell completion support that is actually superior to that provided by click-completion.

Describe the solution you'd like

Remove the click-completion dependency and use click's own shell completion code instead.

Describe alternatives you've considered

Submit patches to click-completion, but the project hasn't seen any commits or merged pull requests since 2019.

Additional context

click-completion has bugs and room for improvement that seem unlikely to get addressed any time soon.