feat: add castArray and castArrayIfExists

[aleclarson]

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Summary

• Both functions return a shallow copy if input is an array.
• If input is nullish, castArray returns a (null | undefined)[] type while castArrayIfExists returns null | undefined.
• Otherwise, both functions return T[] (e.g. string[] if input is a string).

For any code change,

• Related documentation has been updated, if needed
• Related tests have been added or updated, if needed
• Related benchmarks have been added or updated, if needed

Does this PR introduce a breaking change?

No

Bundle impact

Status	File	Size
A	src/casted/castArray.ts	77
A	src/casted/castArrayIfExists.ts	87

[aleclarson]

I'll be renaming these to castArray and castArrayIfExists to align with Lodash.

https://lodash.com/docs/4.17.15#castArray