One2many and Many2many use a special "command" to manipulate the set of
records stored in/linked to the field. There is a total of 7 commands:
create, update, delete, unlink, link, and set.

* The **create** command (id: 0) is used to create a new record in the
  related model and to link it in the current record.
* The **update** command (id: 1) is used to update the targeted record id
  with new values.
* The **delete** command (id: 2) is used to both remove the target record
  id from the related table and to remove its link from the current
  record.
* The **unlink** command (id: 3) is used to remove the link to the
  targeted record id from the current record.
* The **link** command (id: 4) is used to link the targeted record id
  which exists already in the related model in the current record.
* The **clear** command (id: 5) is used to empty the list of linked record
  ids.
* The **set** command (id: 6) is used to replace the list of linked record
  ids with the provided.

Each command is a tuple with up to 3 elements, the second and/or third
element is ignored when it is not applicable to the command:

* the mandatory command identifier
* the targeted record id (1, 2, 3, 4)
* create/write values dictionary (0, 1) or target record ids (6)

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Before this contribution, the several commands did not have a
standardized name, the tuples were just hard-coded with the integer
identifier. For long developers have been complaining the commands were
hard to write and hard to read mainly due to the meaningless integer.

This contribution provide a new namespace with the constants and
functions desired to ease writting and reading the x2many commands. All
modules have been updated to use the new constants instead of the
integer identifier.

Task: 2366606

Description of the issue/feature this PR addresses:

Current behavior before PR:

Desired behavior after PR is merged:




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