[FIX] web_editor: fix anchor range selection

When we double click a form submit anchor, and depending on the selected
element before click, the selection range ("document.getSelection()") can
start from a parent of the targeted node. This will send a wrong range
to be used in LinkDialog widget.

The goal of this commit is to prevent this behaviour by adjusting the range
if it starts from anchor parent.

Note: the bug only occurred in Firefox.

Part of https://github.com/odoo/odoo/pull/62206


task-2381305

closes odoo/odoo#62206

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>