Re: [w3c/selection-api] `Selection.direction`'s behavior should be based on the node's position (Issue #334)

> What do you mean by that? Selection direction is neither forward nor backward when a user selects text with a mouse on macOS, and anchor & focus are not well defined in such cases.

Currently, we have access to the anchor & focus node via the Selection API (`anchorNode`, `focusNode`), but if the selection crosses the shadow boundary, these nodes will be rescoped to the shadow host, therefore we must rely on the `direction` property.

Let me demonstrate the issue in an example with a basic operation of the selection with keyboard (`Shift` + `ArrowLeft`/`ArrowRight`)

We have a text that will be selected with double click, which makes the `direction`'s value to `none`.

```
// within a ShadowRoot
<div cE=true>Lorem |ipsum| dolor</div>
```

The `Shift` + `ArrowLeft` should extend the focus boundary-point, but since we can't determine that anymore due to the inconsistent behavior of the `direction`, we don't know that which boundary-point should be extended.

So, from that point, if I press `Shift` + `ArrowLeft` and update the bounday-point via script, that can be any of the following results:

```
<div cE=true>Lorem| ipsum| dolor</div>
// or
<div cE=true>Lorem |ipsu|m dolor</div>
```

After we changed the selection to any of the scenario above, we still can't rely on the `direction` as to determine the focus boundary-point, since it may not will be updated.

> When the [selection](https://w3c.github.io/selection-api/#dfn-selection)'s [range](https://dom.spec.whatwg.org/#concept-range) is mutated by scripts, e.g. via [selectNode](https://dom.spec.whatwg.org/#dom-range-selectnode)(node), [direction](https://w3c.github.io/selection-api/#dfn-direction) of the [selection](https://w3c.github.io/selection-api/#dfn-selection) must be preserved.

There is an example with comments about the basic implementation of the inline directional navigation with selection, that shows why I have to rely on the "`nodeDirection`".

https://codepen.io/IDontHaveAny/pen/RwzxMRB?editors=1010


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