Change changelog entry format

[pomerantsev]

Change what changelog entries look like.
Right now they’re generated by @changesets/changelog-github, but I don’t like their inflexibility: https://github.com/changesets/changesets/blob/a3acd12146e888abb104a6a41399be8bba0f6169/packages/changelog-github/src/index.ts — should I write my own function for generating those messages or contribute to @changesets?
Preferred format: Description, followed by a link to the pull request in parentheses.