[native] Add spoiler style to native markdown styles

Summary:
We add the styles for spoiler text on native by adding a style for both the text color and the background to make the spoilers uniquely
display.

https://linear.app/comm/issue/ENG-1984/add-spoiler-style-to-native-markdown-styles

Depends on D5347

Test Plan:
Visually works as expected and the spoiler styles apply to only the spoiler text.

Reviewers: atul, ginsu, abosh

Reviewed By: atul, ginsu

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5373