Introduce a CHANGELOG for the wasm node

[tomaka]

After this PR is merged, every noticeable PR that concerns the wasm node should update this CHANGELOG.

I'm not including changes such as #2073 because they only concern the full node.

We could consider adding a CI action that checks whether the CHANGELOG respects a certain format, but I haven't found anything great.
We could also consider adding a CI action that reminds you to add a CHANGELOG entry if you didn't (unless you explicitly say "this doesn't need a changelog entry"), but that also seems kind of complicated.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[github-actions]

twiggy diff report

Difference in .wasm size before and after this pull request.

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[melekes]

👍