Move @types/react-native to Peer Dependencies

[liamfd]

Summary

Moves the @types/react-native dependency to peer dependencies.

For context, this is useful because otherwise, a user of this library with a direct dependency on @types/react-native can wind up with two different versions of the @types/react-native installed, one under node_modules/@types/react-native and one under node_modules/react-native-markdown-renderer/node_modules/@types/react-native. That results in TypeScript errors like:

node_modules/react-native-markdown-renderer/node_modules/@types/react-native/index.d.ts:9488:11 - error TS2451: Cannot redeclare block-scoped variable 'originalXMLHttpRequest'.

9488     const originalXMLHttpRequest: any;
               ~~~~~~~~~~~~~~~~~~~~~~

  node_modules/@types/react-native/index.d.ts:9574:11
    9574     const originalXMLHttpRequest: any;
                   ~~~~~~~~~~~~~~~~~~~~~~
    'originalXMLHttpRequest' was also declared here.

This should resolve #53.

There's another PR for this here: #113, but it appears to make extraneous changes and hasn't seen any responses. I don't mean to step on any toes, just figured this was a more direct way to get a more focused PR up.

I only have a sample size of 1, but this change works in my project, and does not break the existing typing as far as I can tell.

If there's anything I can do to help get this across the line, please let me know.

[scottyeck]

@mientjan Any thoughts on this PR?