Define TypeScript guidelines

[hayata-suenaga]

Part of TS migration issues.

• Create the first version of the guideline in app/contributingGuides/TYPESCRIPT_STYLE.md.
• A rough overview of the guideline is in the design doc.
• Add succinct code examples as needed.
• Create sub-files if necessary to divide the guideline into sections.
• While writing the guideline, if you encounter any unclear points that should be discussed, post them on the #typescript-new-expensify channel.