Resolve #3399036 "Restore dx"

restored source branch 3399036-restore-DX

added 1311 commits

928eba01...93b747d5 - 608 commits from branch project:10.3.x
93b747d5...296e471f - 693 earlier commits
181827af - Issue #3443108 by catch: Optimize AssetResolver caching
aa237b24 - Issue #3443886 by longwave: Remove deprecated code from MarkupInterfaceComparator
4c93088c - Issue #3422915 by magaki, karanpagare, joachim, alexpott, Keshav Patel,...
25de611b - Issue #3443709 by quietone: Enable Drupal.NamingConventions.ValidVariableName.LowerCamelName
c0d8b4bb - Issue #2638686 by solideogloria, garphy, loziju, smustgrave, roderik, Berdir,...
db518452 - Issue #3399036 by godotislate, quietone, Wim Leers, kim.pepper, Keshav Patel,...
7b9f4c99 - Issue #3436490 by mondrake: Method getMockForAbstractClass() of class...
3ac65a2e - Issue #3443915 by plopesc, mherchel: Font for navigation is preloaded in...
dfcc8e8b - Issue #3444028 by andypost: [8.4] Fix implicitly nullable type declarations in core/includes
70e44951 - Aim for original DX, with more precise error messages for YAML-defined...