Add support for Yarn v2

[Basaingeal]

The lockfile that yarn v2 generates is different from the lockfile that yarn v1 generates. Dependabot currently always creates Yarn v1 lockfiles, but yarn v2 can't run yarn install using a v1 yarn lockfile. This is the error message received:

 YAMLException: bad indentation of a mapping entry at line 4, column 15:
       dependencies:
                   ^
       dependencies:
                   ^

[feelepxyz]

@Basaingeal 👋 It's planned but going to be a little while before we get to it. We're focused on integrating Dependabot Preview functionality natively within GitHub so we've had to pause on package manager improvements while we do this.

We're always open to contributions to dependabot-core: https://github.com/dependabot/dependabot-core

[ylemkimon]

For the time being, I'm using a GitHub Actions workflow to fix and update PRs created by Dependabot. See #1297 (comment).

[feelepxyz]

Closing in favour of #1297