feat: separate groups into major/minor/patch (#2426)

Previously if grouping, all major/minor updates within that group were combined into one. Instead, we now honour the “separateMajorMinor”, "separateMinorPatch", and "separateMultipleMajor" settings and keep the groups separate if necessary.

For maximum compatibility with existing PRs, we name branches like `renovate/group-name` whenever possible and only name them like `renovate/major-group-name` or `renovate/patch-group-name` if major or patch are found.

Closes #2425