fix(picker): handle optional reversed last icon in tree lists

[Doekeb]

Description

Pickers that list items in a tree structure (including explore, treesitter, undo, and lsp_symbols) use box-drawing characters to indicate tree hierarchy:

snacks.nvim/lua/snacks/picker/config/defaults.lua

Lines 357 to 361 in dec29f5

	tree = {
	vertical = "│ ",
	middle = "├╴",
	last = "└╴",
	},

The vertical and middle characters exhibit vertical symmetry, but the last character does not. This means that whenever a picker is opened using a layout with reverse = true (e.g. telescope), the last icon points in the wrong direction (see screenshots).

This change addresses this issue by using an optional icons.tree.last_r icon for the last sibling when it is available and the layout has reverse = true. It also adds the appropriate last_r icons to the default config and the undo picker config.

Screenshots

These screenshots show both preserved "good" behavior for non-reversed layouts and fixed "bad" behavior for reversed layouts.

Before	After