Prune babel-loader's cache so it does not grow forever

**I'm submitting a feature request**
Would it be possible to add a way to prune the babel-loader cache after webpack runs? We've started uploading/downloading our cache in CI to help performance across builds, but the babel-cache grows unbounded and eventually bogs down our CI.

**Babel Loader Version**:
7.1.4

**Current behavior:**
babel cache grows unbounded

**Expected/desired behavior:**
It doesn't.

No file or test is named. Start by locating babel-loader's cache handling and the webpack lifecycle point relevant to pruning after a run; define how pruning should work without harming cache reuse, then verify through CI-style builds that the cache no longer grows without bound.

Written by the indexing model from the issue text.