Proposal: add `arrayConcat`

[tychenjiajun]

Array concat has wrong types, so maybe we should add an arrayConcat function as a quick fix.

See microsoft/TypeScript#36554 (comment), microsoft/TypeScript#41913, microsoft/TypeScript#33645 for more use cases.

[sindresorhus]

Sounds good 👍

[tychenjiajun]

Should this be implemented in type-fest first?

[sindresorhus]

Nah. It can just be added here.