Adds the possibility to select directories

[br14n-sol]

Closes #18
Replaces #19, #20 ¹

A new option is added:

type?: 'file' | 'directory' | 'file+directory' // by default: 'file'

• If type is 'file', you can only select files.
• If type is 'directory', you can only select directories.
• If type is 'file+directory', you can select both files and directories.

Other changes:

• From now on, the backspace key will be used to navigate within directories.

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Questions:

• If only directories are expected to be selected, should the files be hidden? ²
• If only files are expected to be selected, should the directories be hidden? ²
• If the directories are hidden, what would navigation be like? ³
  If there are no directories to enter, you could only go backwards in the directory tree, but not forwards.

Footnotes

1. Thanks to @wenshuishi for suggesting this and making the first implementation. ↩

2. These questions will be addressed in the future. ↩ ↩²

3. With the introduction of the ability to filter both files and directories in pr Replace match option with filter #27, the blocking of navigation will not be a priority for the time being. ↩