Add ArraySpeciesCreate, Construct, IsArray and IsInteger abstract operations; Fix %String% intrinsic; Update @@species to return existing `Symbol.species` when available; Bump engine requirement; Add missing semicolons; Update readme

[UltCombo]

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TODO:

• [ ] Write tests

I'm submitting these additions early so you can check whether the approach and style are appropriate.

@domenic By the way, do you prefer (and have time) to review each change before merging, or would you prefer a "commit first, ask questions later" strategy?

[UltCombo]

Looks like I got a bit carried away fixing some nits in the process. Let me know if you would prefer to have split commits or PRs for those.

[domenic]

Hey, this LGTM. Feel free to commit first, ask questions later :). Same with the other PR.

[UltCombo]

Thanks. :smile_cat:

[UltCombo]

I'll write the tests before merging, I'd feel bad lowering a project's code coverage.