Conversion doesn't check types

[Andy Zhou]

Please save me some time and use the following template. In 90% of all issues I can't reproduce the problem because I don't know what exactly you are doing, in which environment, or which y-* version is responsible. Just use the following template even if you think the problem is obvious.

Checklist

• Are you reporting a bug? Use github issues for bug reports and feature requests. For general questions, please use https://discuss.yjs.dev/
• Try to report your issue in the correct repository. Yjs consists of many modules. When in doubt, report it to https://github.com/yjs/yjs/issues/

Describe the bug
A clear and concise description of what the bug is.

When converting from a Prosemirror Node to a YXmlFragment, y-prosemirror doesn't check that attributes it sets are actually strings.

const type = new Y.XmlElement(node.type.name)
for (const key in node.attrs) {
  const val = node.attrs[key]
  if (val !== null && key !== 'ychange') {
    type.setAttribute(key, val)[Kevin Jahns, 2 years ago: • revert toms refactor](https://sourcegraph.com/github.com/yjs/y-prosemirror/-/commit/e39c91e45e113220b5d66311a24c9397b7af0a25)
  }
}

This is problematic because a Prosemirror Node can have a non-string attribute, such as a number. Using YXmlElement.getElement(...) can therefore return a non-string which claims to be a string.

To Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Open console
4. See error

Expected behavior
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Screenshots
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Environment Information

• Browser / Node.js [e.g. Chrome, Firefox, Node.js]
• Yjs version and the versions of the y-* modules you are using [e.g. yjs v13.0.1, y-webrtc v1.2.1]. Use npm ls yjs to find out the exact version you are using.

Additional context
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[Andy Zhou]

Ping on this. I think there's a bunch of options to fix this, but not sure how you think about backwards compatibility.