large scale refactoring of PluginSettings to minimise boilerplate #300

All 101 tests pass in 16s

101 tests +2 101 ✔️ +2 16s ⏱️ +6s
  16 suites ±0     0 💤 ±0
  16 files ±0     0 ❌ ±0

Results for commit 7440640. ± Comparison against earlier commit 5a753bb.