[FIX] *: enable to use the backslash character in a field label of form

*website

Steps to reproduce the bug:
- Drop the "Form" snippet on the website.
- Add a new field.
- Replace the field label by "test\".
- Save.
=> Traceback of type "Cannot read properties of null (reading
'dataset')" appears.

In the `start()` function of the `s_website_form` public widget, the
field names of the form are extracted thanks to the `serializeArray()`
function. The field elements are then found by doing a `querySelector()`
on the field names. In our case, the field name of the new field
extracted by `serializeArray()` is `test\\` (there are two backslashes
because the field name is a string so the first backslash has to be
escaped by a second backslash). The problem is that this string has to
be escaped one more time in order to be used in a `querySelector()`. As
it is not the case, the result of the `querySelector()` is `null`
leading to a traceback when the code tries to access its dataset. To
solve the problem, the character `\` is encoded before being stored in
the `name`, `data-name` or `data-visibility-dependency` attribute of the
field element. Thanks to the encoding, the `querySelector()` operation
can perform correctly on those attributes.

opw-3470291

closes odoo/odoo#134021

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