Skipping Linux and Windows keymaps here, as it's hard to find a
consistent base-binding for all 4 actions across all platforms that
don't break important actions for each keymap. Someone else can think on
that and make a proposal.
Release Notes:
- Added bindings for navigating between sibling nodes in the syntax tree
on macOS (`cmd-ctrl-{up/down}` for `SelectPreviousSyntaxNode` and
`SelectNextSyntaxNode`). Breaking change: the existing syntax tree
parent/child navigation bindings have moved from
`ctrl-shift-{left/right}` to `cmd-ctrl-{left/right}` to create a unified
four-directional navigation pattern where all syntax tree operations use
the same modifier combination. We could not use the previous base
modifiers without breaking more bindings.
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