Editorial: Improve integer index operations

[gibson042]

• Rename StringGetOwnProperty to StringGetIndexPropertyDescriptor
• Rename IsValidIntegerIndex to IntegerIndexedElementExists
• Introduce IsIntegerIndexInRange to support both

[ljharb]

What’s the improvement here?

[gibson042]

The name of "StringGetOwnProperty" suggests that it gets any own property, when in reality it is limited to index properties (and returns a descriptor rather than just a value). Similarly, "IsValidIntegerIndex" suggests a static test of arbitrary integer index properties, when in reality it is limited to those of a specific typed array instance. Both of those are renamed for clarity, and "IsIntegerIndexInRange" is introduced as a generic operation backing them both (basically, what the "IsValidIntegerIndex" name suggests).