Add option to sort buffers by the time they were last accessed

[ChHaeni]

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Buffers can be sorted according to the time they were last accessed (as printed by :ls t)

[generated-files-bot[bot]]

Warning: This pull request is touching the following templated files:

• doc/lualine.txt - lualine.txt is generated from README.md. Make changes there instead.

[ChHaeni]

I'm open to any other option naming other than ls_flags and/or option description.

[ChHaeni]

Many thanks to Jan Henning Thorsen for his suggestion to extend the sorting to any :ls flag!

[jhthorsen]

Sorry for deleting my message, but I did not like the way the buffers were “jumping” around when the sorting changed.

This is unrelated to this PR, but I would rather prefer if I could manually reorder the buffers like you can with bufferline.nvim, but I haven’t found the time to implement that.

[Granddave]

@jhthorsen I would love to be able to reorder buffers!

[ChHaeni]

Closing this PR since #887 addresses buffer sorting in a more generic way.