Allow users to customize the variance of built-in `Array`

Bug Report

🔎 Search Terms

Array covariance hardcoded?, Array invariance, --noLib

🕗 Version & Regression Information

Tested with 4.9.5

⏯ Playground Link

https://tsplay.dev/WPzdYN

💻 Code

// @noLib: true

interface Array<T>
  { [i: number]: T | undefined
  , push: (t: T) => number
  }

interface _Array<T>
  { [i: number]: T | undefined
  , push: (t: T) => number
  }

declare let as: Array<"a">
declare let abs: Array<"a" | "b">
abs = as

declare let _as: _Array<"a">
declare let _abs: _Array<"a" | "b">
_abs = _as

🙁 Actual behavior

The assignment abs = as compiles.

🙂 Expected behavior

The assignment abs = as should not compile, just like the assignment _abs = _as does not compile.

Start by reproducing the TypeScript Playground example linked in the issue, especially the comparison between built-in Array and _Array under @noLib. Trace how the compiler handles variance for the built-in Array, then verify that abs = as is rejected like _abs = _as without changing the demonstrated custom-interface behavior.