Add semanticCommitType awareness to generateBranchConfig()

[rarkins]

Describe the proposed change(s).

Renovate is not aware of the meaning of values of semanticCommitType - they're all just strings. Currently, when a branch has mixed values for semanticCommitType then whichever is the first update in the branch is used as the value for branch semanticCommitType.

Instead, we could:

• Add awareness of the "importance" of semanticCommitType values (e.g. fix is more important than chore)
• Implement this awareness in

  renovate/lib/workers/repository/updates/generate.ts

  Line 162 in f9ab074

  export function generateBranchConfig(

  so that the "most important" semanticCommitType value is chosen from the branch, instead of the first.