Validate minibuffer input with one Enter keypress (not two)

[vindarel]

The completion pop-up requires 1 Enter keypress to select the first entry, and another Enter keypress to validate it. We can shorten this to 1:

(define-keys lem/completion-mode::*completion-mode-keymap*
  ("Return" (cmd
              (lem/completion-mode::completion-select)
              (when (lem/prompt-window::current-prompt-window)
                (lem/prompt-window::prompt-execute)))))

by metnal on Discord