Export MarkedStyles type

[AdamGerthel]

Is your feature request related to a problem? Please describe.
In order to more easily manage merging styles and such before passing them to <Markdown> in a TS environment.

Describe the solution you'd like
I'd like to be able to import and use the MarkedStyles type

Describe alternatives you've considered
I'm currently working around this by using MarkdownProps['styles'] but that's less elegant.

[gmsgowtham]

Released as part of https://github.com/gmsgowtham/react-native-marked/releases/tag/v7.0.0. @AdamGerthel Please let me know if it needs to be backported for your use-case.