[FIX] web_editor: handle arrow keys next to zero-width spaces

Consider the following situation: `<p>a[]b<span><zws></span>c</p>`,
where `<zws>` is a zero-width space and `[]` is the collapsed selection.
On pressing the `ArrowRight` key, we want to enter the `<span>` so we
don't press once to be after the "b" then once again to be before the
`<zws>`. The cases with a non-collapsed selection, and with the left
arrow are analogous. This is what this commit allows us to handle.

X-original-commit: 8e1d5ff3f3bdef8ca2f0eb069c62140260aad3f6
Part-of: odoo/odoo#86930