Subcommands tab-completion

[tuurep]

Followup on #15, which adds the subcommand close, i.e.

• :RegisterEditor close a (close all @a register buffers)

We're following the general idea from these guidelines:

https://github.com/nvim-neorocks/nvim-best-practices?tab=readme-ov-file#white_check_mark-do-1

It would be nice if the subcommand was completable with tab.

Not sure: should we have argument completion? i.e. a list of all possible register names?

• In a way would be ridiculous to have each alphabet a-z 😄

[tuurep]

Yeah, no need for the argument (register name) completion, I think.