wip strict null checks

Author: Basarat Ali Syed

docs/options/strictNullChecks.md

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+## `strictNullChecks`
+By default `null` and `undefined` are assignable to all types in TypeScript e.g.
+
+```ts
+let foo: number = 123;
+foo = null; // Okay
+foo = undefined; // Okay
+```
+
+This is modelled after how a lot of people write JavaScript. However like all things TypeScript allows you to be *explicit* about what *can and cannot be* assigned a `null` or `undefined`.