[FIX] web_editor: fix selection on editable root

Some of the Editor methods relies on not having inline elements at
the editable root. For this reason, the cursor should never be placed
at the editable root, but always inside a block element.

Before this commit, in some situations (notably around tables and
horizontal separators), the cursor could be placed having its anchorNode
at the editable root, allowing the user to insert inlined text at it.

This commit fixes the selection range in such cases, and an empty paragraph
is inserted in cases where there would be no other way to insert text before
or after an existing block.

task-3128747
opw-3106752

Part-of: odoo/odoo#113379