fix @types/googlemaps uses of MapTypeId | string (#38204)
fix @types/googlemaps uses of MapTypeId | string (#38204)

MapTypeId as of #38084 is defined as a string enum, but all uses of
MapTypeId were accidentally left as ``MapTypeId | string`` which looses
type information.

I updated returns to be strictly MapTypeId, but input still seems to need
to be ``MapTypeId | string`` because the TypeScript compiler seems to not
recognize that a string with the proper value is a MapTypeId without
casting it as such. Changing this would break all consumers and does not
seem to make sense to do.
dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped,mcliment/DefinitelyTyped,dsebastien/DefinitelyTyped,georgemarshall/DefinitelyTyped,georgemarshall/DefinitelyTyped