feat: Move @types/react-dom to optional peer dependencies

[eps1lon]

Closes #1184
Docs: testing-library/testing-library-docs#1384

BREAKING CHANGE:
@testing-library/dom was moved to a peer dependency and needs to be explicitly installed. This reduces the chance of having conflicting versions of @testing-library/dom installed that frequently caused bugs when used with @testing-library/user-event.
@types/react-dom needs to be installed if you're typechecking files using @testing-library/react.

Type dependencies need to match their runtime counterpart.
If foo is a dependency, @types/foo needs to be one as well.
If foo is a peer dependency, @types/foo needs to be one as well.
This is especially apparent if the constraint on foo spans multiple major versions.
If we'd make @types/foo a direct dependency, users couldn't control which major version they get.
Package managers would pick the highest.
By moving @types/foo to peer dependencies, users can control which version of @types/foo they have.

In our case specifically, you'd get type mismatches of @types/react-dom if we just widen the peer dependency constraint from ^18 to ^18 | ^19. Currently, we're already compatible with the planned React 19 features so all we'd have to do is widen the constraint on release.

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[MatanBobi]

Thanks.
Just a thought since we're doing this and bumping a major - lately we're having many issues with DTL version mismatches in RTL and user-event, do we maybe want to think about also move DTL to peerDeps?

[eps1lon]

do we maybe want to think about also move DTL to peerDeps?

I think so. Since DTL is stateful due to the config object, having multiple versions is problematic.

Ideally we wouldn't have the stateful config object to begin with but that requires a large refactor that needs coordination.

We don't need to rush this in this PR though. Could you help out and prepare docs and a commit making that change?

[MatanBobi]

We don't need to rush this in this PR though. Could you help out and prepare docs and a commit making that change?

Sure thing, I'll get to it later this week.

[MatanBobi]

@eps1lon Wdyt about making this change as part of a major too? #1277

Also, here's the PR to move DTL to peer: #1309

[eps1lon]

Also, here's the PR to move DTL to peer: #1309

Just push this commit to this branch. I don't want to go through the alpha branch shenanigans and just ship the major with one PR.

[MatanBobi]

Just push this commit to this branch. I don't want to go through the alpha branch shenanigans and just ship the major with one PR.

Done. I haven't fixed the conflicts though as we have more changes coming to that file.