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

[lozcalver]

Description

SS5 replaced $First and $Last with $IsFirst and $IsLast, but this change wasn’t reflected in elemental

Additional context or points of discussion

No response

Validations

• You intend to implement the feature yourself
• You have read the contributing guide
• You strongly believe this feature should be in core, rather than being its own community module
• You have checked for existing issues or pull requests related to this feature (and didn't find any)

PRs

• Add IsFirst/IsLast methods to match SS5 conventions (closes #1274) #1275
• DOC Add changelog item for IsFirst/IsLast in elemental developer-docs#631