IntegerItem 'size' setting should be a storage setting

Patch.

Test added.

Side note: #2368349: Entity view and form display configuration schemas are too verbose / key ones missing adds a missing config schema for the current *field* setting.
If it gets committed first, the patch here will need to be updated to change the config schema accordingly (-> storage setting)

Side note: It's just the "test only" patch that failed, as it should.

Actually, #2368349: Entity view and form display configuration schemas are too verbose / key ones missing landed, so as mentioned in #4, we now also need to update the config schema.

[edit: heh, crosspost]

Bleh. Never mind #10, something weird happened in the copy/paste.

Patch with config schema change, and interdiff with #3
(also, using a diff algorithm that better reflects the actual change)

"SchemaIncompleteException: Schema errors for field.storage.entity_test.h7quxlds with the following errors: field.storage.entity_test.h7quxlds:settings.unsigned missing schema"
Weird - 'unsigned' is indeed missing from the schema (#2370305: Refactor field type configuration schemas for DX, easier to find errors is fixing that), but that doesn't cause an exception in NumberItemTest on my setup.

Well, adding it here as well if that's what it takes to make the testbot happy - this patch and #2370305: Refactor field type configuration schemas for DX, easier to find errors will conflict anyway.