Migrate to `purs-changelog` for handling changelog file

[JordanMartinez]

Description of the change

Uses purs-changelog to handle the changelog for this repo. (This tool is a port of the update-changelog.hs script used in the PureScript repo). The benefit of this change is that we will no longer get merge conflicts in the CHANGELOG.md file when merging multiple PRs in a short time period. To see an example of what using purs-changelog for updating a changelog looks like, see this commit

Moreover, the first checkbox in the PR template list below will no longer require authors to indicate the PR number (so, no more "open a PR, see what the PR number is, and push a commit for that number).

Lastly, when we need to update the changelog for a new release, one only needs to run purs-changelog update so long as the URL for the origin remote name (i.e. git remote -v) refers to this repo and not a fork.

By making all core, contrib, web, and node libraries use this tool, in the future we can use GitHub's API to produce a single file that lists all the changes made in an ecosystem update (e.g. v0.15.x release).

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Checklist:

• [ ] Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• [ ] Linked any existing issues or proposals that this pull request should close
• [ ] Updated or added relevant documentation
• [ ] Added a test for the contribution (if applicable)

[JordanMartinez]

My original goal for the purs-changelog project was to be used in single projects, not across whole orgs. I agree with what you're saying here in that we should have a centralized place for documentation. However, I'm now wondering how I should incorporate that into the design of the tool since that wasn't something I had considered. Moreover, one point you mentioned against doing this was doing this in contrib libraries first as a sort of pilot program before we move it to core in case there are any issues with the code.

So, for now, I'm going to close this PR.