add support for bumping version of `packageManager` specified inside `package.json`

I'd like to be able to use `yarn` or `pnpm` as an alternative package manager. See docs: https://nodejs.org/api/corepack.html

To do so, they get configured in `package.json` under the `packageManager` field - with a version specified. e.g. `"packageManager": "yarn@1.22.15"`

It would be great if Dependabot could keep that version up to date - as currently it does not.

Start by tracing how Dependabot reads package.json dependency metadata and how its existing package-manager update support is organized; the issue provides the packageManager field format and Corepack documentation as the starting point. Done means a yarn@ or pnpm@ version in package.json is detected and kept up to date, with the relevant behavior covered by the project's existing tests.

Written by the indexing model from the issue text.