Add IsFirst/IsLast methods to match SS5 conventions (closes #1274)

[kinglozzer]

Description

Deprecates First() and Last() in favour of IsFirst() and IsLast(), as these methods are intended to override the built-in iteration methods of those names.

Manual testing steps

Check the value of $IsFirst and $IsLast in elemental block templates

Issues

• BaseElement::First() / Last() should be IsFirst() and IsLast() #1274

Pull request checklist

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[GuySartorelli]

Makes sense, thanks for submitting this. I've filled in the missing information from the PR template for you this time around.

Please also add a PHPDoc comment describing what the isFirst() and isLast() methods do.
Please also make a PR to developer-docs adding notes about these new deprecations to the 5.4.0 changelog.

[GuySartorelli]

Please also add either the API or NEW prefix to the commit message.

[GuySartorelli]

There are tests for the original methods. Please add tests for the new methods as well.

[kinglozzer]

Have made the requested amends, docs PR: silverstripe/developer-docs#631

[GuySartorelli]

Suggested change

	$this->assertFalse($element->Last());
	$this->assertTrue($element2->Last());
	$this->assertFalse($element->IsLast());
	$this->assertTrue($element2->IsLast());

[GuySartorelli]

Suggested change

* @return bool

Sorry, should have caught this last time. We don't need the PHPDoc if it doesn't give any additional context over the actual type hint.

[GuySartorelli]

Suggested change

* @return bool