[FR] Compile this library in strict mode

[vicb]

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)

[amuramoto]

Would love to see this happen