Hello!

While trying to speed up tests for a larger project, I noticed that having to initialize extend-expect for every test adds quite a bit to the overall test run-time. This is about 5-10% for us, but I think it could be bigger for other projects with less overhead.
This would of-course only benefits tests that don't use jest-dom, but at least in our case only about 25% of tests used jest-dom.

Also trying to add jest-dom to a bigger test suite could slow existing tests down and prevent adoption, while lazy-loading could minimize the impact.

A solution would probably depend on importing the matchers withrequire only when a thin module is already loaded.
This could be added as a separate file to import insetupTests, eg@test-library/jest-dom/lazy

The maintenance cost of this would be having to add every new matcher to the list of matchers in the lazy loader.

I would be happy to contribute this change if this makes sense to the maintainers