Elements should be selectable via keyboard like a letter would

Given

```
text
[an element]
text
```
it’s possible to place a caret in the first line and shift+end to select from caret till the end of doc.

But it’s not possible to go selecting letter-by-letter (using shift+RrightArrow) or word-by-word (using shift+Alt+RrightArrow). It should be possible. And element should behave like a letter (one press to select it all), but also like a word (one press to select it all too).

Start by reproducing the issue in the ProseMirror editor with an element between text lines, using Shift+Right Arrow and Shift+Alt+Right Arrow. Trace the keyboard selection handling for elements; done means an element can be selected with either letter-by-letter or word-by-word selection and behaves as one unit.

Written by the indexing model from the issue text.