Better handling of completions when opening a file

Description

As discussed in #2891 the current behaviour of completions for opening a file is frustrating when dealing with directories. There is no currently no straightforward way to select a directory without finishing the completion. Pressing Enter when the directory is highlighted leads to micro trying to open the directory and receiving the Error: X is a directory and cannot be opened message.

As requested, this issue is to discuss ways in which this could improved. My preference would be to adopt the zsh behaviour, whereby pressing either / or Enter when a directory is highlighted only adds that directory to the current target path, instead of attempting to open it. The user can then press tab again to attempt completion within the sub-directory the path points to.

Does anyone else have another suggestion?

Start by reproducing completion while opening a directory in micro, including pressing Enter or / when the directory is highlighted. Read the completion and file-opening paths to determine where the directory is treated as an open target. Done requires an agreed interaction design, with the issue's preferred behavior validated for continuing completion inside the selected directory.