feat(builtin.buffers): enhance and bind delete_buffer action

[jamestrew]

Binds the delete_buffer action to <M-d> by default for the buffers pickers.

closes #3128

Also enhances the delete_buffer action to have a "cleaner" behavior for deleting the current buffer (the buffer under telescope). Previously, when deleting the current buffer, the buffer would be removed from the buflist but closing the picker would put you right back into the buffer you just deleted.
From :h bdelete:

If buffer [N] is the current buffer, another buffer will be displayed instead. This is the most recent entry in the jump list that points into a loaded buffer.

Since the telescope prompt buffer is technically the current buffer, this "another buffer will be displayed behavior" wasn't actually triggering.
With this newest change, Telescope will manually get the buffer of the most recent entry in the jumplist and change the window behind telescope to this new buffer.

closes #2643

[Conni2461]

LGTM, besides docs nit

[Conni2461]

Thanks :)

[jamestrew]

Thanks, I agree about setting default mappings but it's such a common question I see here/reddit/discord so.
Switched it to <M-d> since I forgot that <C-x> is already a default mapping.

[igorlfs]

Hey! This commit introduced a bug :(

I can no longer use :Telescope buffers. I get a stacktrace that points to
telescope.nvim/lua/telescope/mappings.lua:330: Attach mappings must always return a value. `true` means use default mappings, `false` means only use attached mappings. So I guess this should be retuning true?

EDIT: I went ahead and made a PR, see below