fix: handle iter_matches node arrays from Neovim 0.12+

[IjonTichyRaumpilot]

Summary

Neovim 0.12 changed the behavior of iter_matches: each capture now returns a list of nodes instead of a single node. This breaks the treesitter backend in aerial with errors like:

attempt to call method 'start' (a nil value)
  ...aerial.nvim/lua/aerial/backends/treesitter/helpers.lua:13
attempt to call method 'type' (a nil value)
  ...aerial.nvim/lua/aerial/backends/treesitter/extensions.lua:118

Fix

Normalize the node value in the iter_matches loop in init.lua by unwrapping the table when one is returned:

if type(node) == "table" then
  node = node[#node]
end

This matches the previous single-node behavior. The existing all = false workaround in the iter_matches call (from #407) is no longer sufficient with Neovim 0.12.

Test

Verified on Neovim v0.12.3 — opening any file no longer produces treesitter backend errors.

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