TypeScript: migrate dom-ready package to TS

[kushagra-goyal-14]

What?

Part of #67691

Migrating the dom-ready package to TypeScript.

Why?

To enhance DX and add type safety.

How?

By porting the code and tests to TypeScript, inlining the () => void callback type and removing the obsolete @typedef Callback.

Since Doctrine was removed from docgen in PR #28615, the tooling now understands inline function types. This completes the original TODO to drop the Callback typedef and use () => void directly in JSDoc and signatures.

Testing Instructions

Typecheck and unit tests.

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[github-actions]

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[kushagra-goyal-14]

Seems like #67671 has already migrated this package to TS and is open, thus closing this PR