`semversioner check` should assert that the current branch contains a change document

[darthtrevino]

Currently semversioner check asserts that a change document has been created. And initially it works great for that, but imagine this flow:

In PR 1, Cyril makes low-impact changes. semversioner check catches that he needs a change document. Cyril uses semversioner add-change -t patch -d ...

In PR2, Pat makes a higher-impact change. They forget to commit a change document, but semversioner check will pass, because a change document exists from Cyril's PR.

Ideally, semversioner check would cause Pat's PR to fail as well because their changes have semver impact that should be documented independently.

[darthtrevino]

Here's how the Yarn Version Plugin is doing this:

Determining Git Changes: https://github.com/yarnpkg/berry/blob/578d8961358503ec04705d15f89284664fe1be61/packages/plugin-git/sources/gitUtils.ts#L313

Determine Version File for Branch: https://github.com/yarnpkg/berry/blob/578d8961358503ec04705d15f89284664fe1be61/packages/plugin-version/sources/versionUtils.ts#L165

Version Check Command: https://github.com/yarnpkg/berry/blob/578d8961358503ec04705d15f89284664fe1be61/packages/plugin-version/sources/commands/version/check.tsx#L351