[FR] Compile this library in strict mode

There are a few typing issues in the code

i.e.

https://github.com/googlemaps/js-markerclusterer/blob/e8eecf49b1d1f59579f3316c705f2c131b796560/src/algorithms/grid.ts#LL49C1-L61C4

`this.state = { zoom: null };` is not compatible with `protected state: { zoom: number };`

This lib should be compiled with `{ strict: true}` in the `tsconfig.json`.

WDYT @amuramoto ?

(I could volunteer to help)