Ability to use arrays in templates

[emteknetnz]

Related to #11196

In this comment it was noted converting arrays to ArrayList at the template level is hard

This PR does do it, though it's done within ViewableData::obj(), which was considered to be a bit too risky

Options provided were:

1. Update ViewableData::obj() to return arrays directly if there are any
2. Update ViewableData::obj() to wrap arrays in either a list or ArrayData depending on what type of array they are
3. Update ArrayList to either never wrap arrays, or to always wrap them in either a list or ArrayData depending on what type of array they are
4. Some combination of the above
5. Decoupling the logic from ViewableData - though that may be more effort than it's worth
6. Something else neither of us considered yet, if someone else has any ideas.

Acceptance criteria

• Native arrays can be returned in PHP methods, and then iterated over in templates
• Nested arrays can be iterated over at all levels
• Changes outside the templating system are kept to a minimum
• Changes outside the templating system are evaluated to see how wide-reaching their affect might be, and the changelog mentions anything worth mentioning
• Associative array can be access via their keys.

PRs

kitchen sink CI run with the main framework PR installed (note the one CI failure is just because pulls/6/arrays-in-templates isn't identified as matching a supported version of Silverstripe CMS - it's only failing because we are using a fork in CI)

• ENH Looping through arrays in templates #11244
• DOC Document ability to loop over arrays in templates developer-docs#517 (CMS 6)

Alternate PR

• ENH Convert arrays displayed in templates to ViewableData #11256