Change `three` to a peer dependency

## Use Case

Having multiple versions of `three` usually causes conflicts, because `three` expects only a single version to be installed. `vole-core` requires `three`, but users of `vole-core` often need to install `three` as well.

From what I understand, adding `three` as a peer dependency could help throw more explicit warnings if incompatible versions are installed! (Though A. I could be wrong and B. `three` doesn't use semver...)

## Solution

- Add `three` to the list of peer dependencies in `package.json`.

## Alternatives

_Please describe any alternatives you've considered, even if you've dismissed them_