Strongly type FieldList

[maxime-rainville]

Many methods on FieldList will silently fail if you give them bad types.

e.g.: $fields->addFieldsToTab('Root.Main', $field1, $field2, $field3) will only add $field1 to the tab.

Acceptance criteria

• Methods in FieldList that are not covered by other interfaces are strongly typed and have an explicit return type.
• addFieldsToTab hard fails if the second parameter is not an array.
• All of kitchen sink is shown to pass CI before any PRs are merged

Shared CI workflow run

• https://github.com/emteknetnz/recipe-kitchen-sink/actions/runs/9073256392

PRs

• API Strongly type Fieldlist #11235
• FIX Pass arrays to addFieldsToTab silverstripe-userforms#1286
• FIX Pass arrays to addFieldsToTab symbiote/silverstripe-advancedworkflow#529
• DOC FieldList is now strongly typed developer-docs#510
• API Deprecate passing non-arrays to FieldList methods #11236 (CMS 5 deprecation)

[GuySartorelli]

FieldList is not in a vacuum - it's the subclass of ArrayList which is a subclass of ViewableData, uses a trait, and implements several interfaces.

The ACs should specify whether any of that is in scope, or if this card is only for methods defined directly in FieldList, or if it's mostly about methods defined directly in FieldList but the implementor can choose to strongly-type other methods if they feel it's relevant to the goal of the card (i.e. making it clearer what can/cannot be passed into FieldList).