Provide comprehensive guidance to convert files to typescript.

[DDDDDanica]

Help shared libraries team on the delivery of these issues:

1. Complete TypeScript guidelines contributor-docs#69
2. Add entry forany, as usage in TypeScript Guidelines doc contributor-docs#47
3. Add TS guideline: "Document safe or necessary uses of type assertions" contributor-docs#57

[DDDDDanica]

https://hackmd.io/hhoOcWhCS-moM7hcE7lntQ

[MajorLift]

I assume that the above tickets were chosen to help describe how the end result of a TypeScript migration should look like. However, the guidelines being written by Shared Libraries are intended for established, mature TypeScript codebases, and therefore either contradict advice that would be appropriate for a migration guideline, or have a different focus altogether:

• any is generally a useful tool to be actively used during migration, not absolutely avoided like in an established TypeScript codebase.
• Type annotations will be very prevalent in a migrating codebase, not minimized as we recommend for TypeScript codebases.
• The migration guide should also have a much larger focus on tooling and process with very different configurations than would be used by a TypeScript codebase.

It might be best to write the migration guide with a specific focus on the needs of a JavaScript codebase being converted, while the TypeScript guidelines focuses on the needs of a mature TypeScript codebase. Then we can examine if there's any overlapping content that could be consolidated or cross-referenced.