Support supplying directories of files in the personal/preload directory

[levinem]

Currently the personal/preload directory accepts .el files. This PR allows you to include directories in the personal/preload directory, and all .el files recursively found in those directories will be loaded as well.

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by checking the relevant checkboxes):

• The commits are consistent with our contribution guidelines
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the user manual (if adding/changing user-visible functionality like modules, commands, configuration options, etc)

Thanks!

[bbatsov]

Why not just modify the previous block instead of introducing a new one?

[levinem]

Why not just modify the previous block instead of introducing a new one?

Do you mean as an or condition with when (file-exists-p? If so, I assumed the mapc command on directory-files could fail if file-exists-p is false but file-directory-p is true. My assumption could definitely be wrong though.

[bbatsov]

Yeah, that's fair, but you can just use file-directory-p in the previous when, as looking at the code it assumes it's always a directory anyways.

[levinem]

Excellent idea! I updated the previous block based on your suggestion.

[github-actions]

This pull request has been automatically marked as stale because it has not had any recent activity. It will be closed soon if no further activity occurs. Thank you for your contribution and understanding!

[levinem]

There seem to be a lot of linter errors for the CONFIGURATION.md file related to line length unrelated to my changes.