Wrap JSDOM to count DOM mutations

[Rich-Harris]

Opening this so we don't lost @btakita's idea in #1274:

Perhaps one way to test performance regressions is to add a wrapper to the JSDOM api which records DOM operations. Then the test can assert n DOM operations were performed.

[marvinhagemeister]

@Rich-Harris Had the same use case a while ago: https://github.com/marvinhagemeister/dom-callcount

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[Rich-Harris]

Closing as we have much more visibility into this stuff in the Svelte 5 world