Adds a Form view object intended to be used in test, which *should* (barring bugs) properly:

* perform default_get and subsequent onchanges when creating a new record
* properly call onchanges when altering fields
* reject changes to readonly fields
* not save unchanged & readonly fields
* require view-required fields
* correctly handle modifiers
* support o2m fields including `*_view_ref` hooks
* should correctly handle the "edition reference" (editable list v form)

M2M support limited to adding & removing (pre-existing) records, I think the developer can just use a separate Form if they want to create/edit those via specific views?

Example:
```python
# create a pseudo form in creation mode (empty recordset) using the specified view for e.g. readonly/required modifiers, valid fields, …
c = Form(self.env['account.voucher'], view='account_voucher.view_sale_receipt_form')
# basic fields are set directly
c.partner_id = partner_id
c.journal_id = sales_journal_id
# for o2m, "create" or "edit" a line and set its fields
with c.line_ids.new() as line:
    line.name = "Voucher for Axelor"
    line.account_id = account_receivable_id
    line.price_unit = 1000.0
    # the line is saved (in the parent, not the server) at the end of the context
# save() returns the record created (or edited, though it's not really
# necessary for an edition you can just reuse the original record)
account_voucher_customer = c.save()
```

TODO: nice docstring?

Unresoved questions:

* should `invisible` modifiers have an effect? Possibly a variant of readonly (can't edit but properly sent to the server)?
* should buttons be supported directly via the form view? Unclear how they'd be invoked since the button name can be e.g. an action id.

Replaces #20894