esnext Array flatten should be flat

[sambostock]

The esnext type declarations for Array should be updated, as per the updated proposal. flatten has been renamed to flat.

TypeScript Version: 3.0.0-dev.201xxxxx

Search Terms:

• flat flatten

Expected behaviour:

Types should be declared for flat on Array.

Actual behaviour:

Types are declared for flatten on Array.

Related Issues:

• feat(es2018): add definitions for Array.prototype.flatten and Array.prototype.flatMap #20431

[mhegazy]

thanks @sambostock!