Add nvim-tree.diagnostics.placement config option

[serhez]

Is your feature request related to a problem? Please describe.
No.

Describe the solution you'd like
It would be nice to have an option in the config such as nvim-tree.diagnostics.placement, which would act similarly to nvim-tree.renderer.icons.git_placement, with possible values left and right. This would allow the user to place diagnostic icons at the left/right end of the nvim-tree window; currently, this defaults to the left end of the window and cannot be changed (AFAIK).

Describe alternatives you've considered
N/A.

Additional context
N/A.

[alex-courtis]

This will be useful and be consistent with git behaviour.

Default value signcolumn for current behaviour.

[alex-courtis]

Ensure we skip empty as per #1952

[alex-courtis]

Fixed at #2396