Closes #21054

## Summary
• make the JetBrains base keymap use subword motions for
`Alt+Left/Right` and `Shift+Alt+Left/Right` in editors
• keep Zed's default keymaps and the underlying `word`/`subword`
primitives unchanged
 • document word vs. subword navigation in the key bindings docs
• document the JetBrains default in the IntelliJ, WebStorm, PyCharm, and
RustRover migration guides

## Testing
 • `./script/check-keymaps`
 • `cargo fmt --all -- --check`
 • `./script/clippy -p editor`
• `cd docs && pnpm dlx prettier@3.5.0 src/key-bindings.md
src/migrate/intellij.md src/migrate/webstorm.md src/migrate/pycharm.md
src/migrate/rustrover.md --check`

Related to #12816 and #34090, but does not actually address the
configurable word separators or broader subword semantics. Intentionally
scoped to JetBrains keymap defaults & docs.

Release Notes:

- Improved JetBrains keymap behavior by adding CamelHump-style subword
navigation in editors.

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Co-authored-by: Tom Houlé <tom@tomhoule.com>
Co-authored-by: Antonio Scandurra <me@as-cii.com>