This PR adjusts the implementation of handling navigational shortcuts (i.e. arrow keys) on `MenuAnchor` and `DropdownMenu`.
## Motivation
The direct outcome of this PR is to allow keyboard to enter submenus on Web: When the focus is on a `MenuAnchor` while the menu is open, pressing arrow keys should move the focus to the menu item.
* Before the PR, this works for all platforms but Web, a problem described in https://github.com/flutter/flutter/issues/119532#issuecomment-2274705565.
It is caused by the fact that `MenuAnchor` does not wrap itself with a `Shortcuts`, and therefore key events when the focus is on a `MenuAnchor` has been working only because the event falls back to the `Shortcuts` widget defined by `WidgetsApp`, whose default value happens to satisfy `MenuAnchor`'s needs - except on Web where arrow keys are defined to scroll instead of traverse.
Instead of defining this problem as "just a patch for Web", I think it's better to define it as a problem of all platforms: `MenuAnchor`'s shortcuts should be independent of `WidgetsApp.shortcuts`. Because even if `WidgetsApp.shortcuts` is redefined as something else, people should probably still expect arrow keys to work on `MenuAnchor`.
Therefore this PR makes `MenuAnchor` produce a `Shortcuts` by itself.
### Dropdown menu
The fix above breaks `DropdownMenu`. `DropdownMenu` uses `MenuAnchor`, while defining its own shortcuts because, when filter is enabled:
* The left and right arrow keys need to move the text carets instead
* The up and down arrow keys need to "fake" directional navigation - the focus needs to stay on the text field, while some menu item is highlighted as if it is focused.
Before the PR, `DropdownMenu` defines these shortcuts out of `MenuAnchor`. In order for the `DropdownMenu`'s shortcuts to take priority, these shortcuts are moved to between `MenuAnchor` and the `Textfield`.
A test is added to verify that the left/right keys move text carets.
Below are psuedo-widget-trees after the PR:
```
MenuAnchor
|- Shortcuts(arrows->DirectionalFocusIntent)
|- MenuAnchor.child
|- menu
DropdownMenu
|- Actions(DirectionalFocusIntent->_dropdownMenuNavigation)
|- MenuAnchor
|- Shortcuts(arrows->DirectionalFocusIntent)
|- Shortcuts(leftright->ExtendSelectionByCharacterIntent, updown->_dropdownMenuArrowIntent)
| |- TextField
| |- EditableText
| |- Actions(DirectionalFocusIntent->DirectionalFocusAction.forTextField)
|- menu
```
## Known issues
After this PR, traversing the menu still have quite a few problems, which are left for other PRs.
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