[core] feat(Dialog): add containerRef prop

[dlichen]

Fixes #5273

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

Adds a ref prop to dialog.

Reviewers should focus on:

I'm not entirely sure if this is the correct approach, also not completely sure how to test, so any feedback is welcomed.

Screenshot

n/a

[adidahiya]

Please add a unit test. You should be able to add a test for this in core/test/dialog/dialogTests.tsx... something like:

    it("supports ref objects attached to container", () => {
        const containerRef = React.createRef<HTMLDivElement>();
        mount(
            <Dialog containerRef={containerRef} usePortal={false}>
                body
            </Dialog>,
        );
        assert.isTrue(containerRef.current?.classList.contains(Classes.DIALOG_CONTAINER));
    });

[dlichen]

Please add a unit test. You should be able to add a test for this in core/test/dialog/dialogTests.tsx... something like:

    it("supports ref objects attached to container", () => {
        const containerRef = React.createRef<HTMLDivElement>();
        mount(
            <Dialog containerRef={containerRef} usePortal={false}>
                body
            </Dialog>,
        );
        assert.isTrue(containerRef.current?.classList.contains(Classes.DIALOG_CONTAINER));
    });

added the test, but still need to test locally. can't seem to figure out yarn test. can hold off on reviewing