TypeScript: Fix and improve types for private-apis

[manzoorwanijk]

What?

This PR converts private-apis package to TypeScript.

Why?

Since private-apis package is used by many other packages, converting this package to TypeScript will pave the way for fixing types on other packages like data and core-data

How?

The PR simply renamed the .js files to .ts and adds the required types to the functions etc.

Testing Instructions

• Just verify that the build and tests are successful

Testing Instructions for Keyboard

Screenshots or screencast

[github-actions]

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[tyxla]

Thanks! We can clean up the pre-existing JSDoc types, just like in #66669.

[tyxla]

LGTM 🚀 Thanks @manzoorwanijk 🙌

[jsnajdr]

A little better type would be unlock<T>(object: unknown): T because:

• the function can safely work with any object and try to unlock it. We might run into problems with the Record type if the locked object doesn't have a compatible type. That will probably happen a lot as we migrate the poorly-typed-yet codebase.
• we should be able to cast the return value to be an object of known type with private methods.

I predict that we'll run into these issues as we use the typed lock/unlock and will eventually come back to revisit this.

[manzoorwanijk]

Updated in #66682

[manzoorwanijk]

That signature seems to break a lot of existing poorly typed usage of lock and unlock