Simpler implementation of dirpreviews

[joelim-work]

From discussion in #1949 (comment)

This is a rewrite of the dirpreviews feature, which was originally implemented in #842. This has a number of improvements:

• The original channel/goroutine for handling file previews is now used for directory previews as well, instead of creating a new dedicated channel and goroutine just to handle directory previews.
• The code for handling previews is now no longer duplicated as a result of the above, which makes maintenance easier
• The loading property of a dir struct now properly indicates whether it has been read from the filesystem, and is not confused with whether its preview has loaded or not.

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For the purposes of release notes, technically there is a bug fix in this change:

• The previewer script is now only invoked for the current directory (not all directories), when starting lf with dirpreviews enabled.