Fix extend selection shortcuts in JetBrains keymap on macOS (#22814)

Fixups https://github.com/zed-industries/zed/pull/20199

As mentioned in [the post-merge comment][1], the original change was
wrong. The JetBrains IDEs use <kbd>⌥</kbd> (option) key on macOS for the
shortcuts, which corresponds to the <kbd>alt</kbd> key in the keymap
config.

Release Notes:

- Fixed extend/shrink selection in JetBrains keymap on macOS

[1]:
https://github.com/zed-industries/zed/pull/20199#issuecomment-2468136572

assets/keymaps/macos/jetbrains.json

@@ -24,8 +24,8 @@
       "ctrl-g": ["editor::SelectNext", { "replace_newest": false }],
       "ctrl-cmd-g": ["editor::SelectPrevious", { "replace_newest": false }],
       "cmd-/": ["editor::ToggleComments", { "advance_downwards": true }],
-      "cmd-up": "editor::SelectLargerSyntaxNode",
-      "cmd-down": "editor::SelectSmallerSyntaxNode",
+      "alt-up": "editor::SelectLargerSyntaxNode",
+      "alt-down": "editor::SelectSmallerSyntaxNode",
       "shift-alt-up": "editor::MoveLineUp",
       "shift-alt-down": "editor::MoveLineDown",
       "cmd-alt-l": "editor::Format",