[display] support toplevel completion after abstract from / to

Similar to completion after `extends` and `implements` for classes, completion on `from |` and `to |` should be allowed. However, right now that fails with "no completion point found".

```haxe
package cases;

class Issue7137 extends DisplayTestCase {
/**
abstract Foo1(Int) from {-1-} {}
abstract Foo2(Int) to {-2-} {}

class Main {
static function main() {}
}
**/
function test() {
eq(true, hasToplevel(toplevel(pos(1)), "Type", "Float"));
eq(true, hasToplevel(toplevel(pos(2)), "Type", "Float"));
}
}
```