Consistently map syntax node navigation actions on macOS (#37874)

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.

assets/keymaps/default-linux.json

@@ -493,8 +493,8 @@
       "alt-down": "editor::MoveLineDown",
       "ctrl-alt-shift-up": "editor::DuplicateLineUp",
       "ctrl-alt-shift-down": "editor::DuplicateLineDown",
-      "alt-shift-right": "editor::SelectLargerSyntaxNode", // Expand Selection
-      "alt-shift-left": "editor::SelectSmallerSyntaxNode", // Shrink Selection
+      "alt-shift-right": "editor::SelectLargerSyntaxNode", // Expand selection
+      "alt-shift-left": "editor::SelectSmallerSyntaxNode", // Shrink selection
       "ctrl-shift-l": "editor::SelectAllMatches", // Select all occurrences of current selection
       "ctrl-f2": "editor::SelectAllMatches", // Select all occurrences of current word
       "ctrl-d": ["editor::SelectNext", { "replace_newest": false }], // editor.action.addSelectionToNextFindMatch  / find_under_expand