Fix FastImage.cache properties in TypeScript types.

[keima]

Hi,
I love this lib.
I occurred compile error following:

node_modules/react-native-fast-image/src/index.d.ts:129:30 - error TS2694: Namespace 'FastImage' has no exported member 'cache'.

129         cacheOnly: FastImage.cache.cacheOnly
                                 ~~~~~

node_modules/react-native-fast-image/src/index.d.ts:130:30 - error TS2694: Namespace 'FastImage' has no exported member 'cache'.

130         immutable: FastImage.cache.immutable
                                 ~~~~~

node_modules/react-native-fast-image/src/index.d.ts:131:24 - error TS2694: Namespace 'FastImage' has no exported member 'cache'.

131         web: FastImage.cache.web
                           ~~~~~

So I had written this patch. Thanks.

[DylanVann]

Looks great, thank you!