Add missing Array.prototype methods (find, fill, findIndex, etc.)

[memburg]

Summary

Implement the remaining commonly-used Array.prototype methods that are currently absent.

Motivation

~25 Array methods are already implemented (map, filter, reduce, forEach, flat, flatMap, splice, etc.). Filling the gaps with the most practical missing methods improves compatibility with real-world JS code.

Priority order (by practical usage)

High:

• fill(value, start?, end?)
• find(predicate, thisArg?)
• findIndex(predicate, thisArg?)

Medium:

• entries() — returns an iterator (can be a simple array of [index, value] pairs)
• keys() — returns an iterator
• values() — returns an iterator
• copyWithin(target, start, end?)
• reduceRight(callback, initialValue?)

Low:

• findLast(predicate, thisArg?)
• findLastIndex(predicate, thisArg?)
• Array.from(arrayLike)
• Array.fromAsync(arrayLike) — requires async support

Out of scope

• toReversed, toSorted, toSpliced, with (ES2023 immutable variants — lower priority)
• toLocaleString