[FIX] web_editor: properly apply/unapply underline and strikeThrough

Underline and strikeThrough styles are special beasts in that they both
are applied to the `text-decoration-line` property. Unlike `font-weight`
or `color`, They behave like a border: setting
`text-decoration-line: none` to an element won't undo the value of the
parent and therefore the text will still show the decoration applied to
its parent. Similarly, nested decorations will apply over each other:
a text node whose parent has an underline decoration and whose
grandparent has a line-through decoration will be both underlined _and_
struck through.
Consequently, we can't treat these styles the same way we treat bold,
italic and colors:
1. To undo the style, we have to split the text from its styled ancestor
2. To apply eg underline to a struck through text, we have to insert a
   new span.

task-2778416

X-original-commit: 93102a7dcb3e2c1a5803ab99f2f924b0b24e44c2
Part-of: odoo/odoo#86930