Allow cross-tyvar defaulting proposals from plugins

compiler/GHC/Tc/Errors.hs

compiler/GHC/Tc/Errors/Ppr.hs

compiler/GHC/Tc/Errors/Types.hs

compiler/GHC/Tc/Instance/FunDeps.hs

compiler/GHC/Tc/Solver.hs

compiler/GHC/Tc/Solver/Monad.hs

compiler/GHC/Tc/Types.hs

compiler/GHC/Types/Error/Codes.hs

docs/users_guide/9.10.1-notes.rst

docs/users_guide/extending_ghc.rst

testsuite/tests/plugins/Makefile

testsuite/tests/plugins/T23832.hs

testsuite/tests/plugins/T23832_invalid.hs

testsuite/tests/plugins/T23832_invalid.stderr

testsuite/tests/plugins/all.T

...te/tests/plugins/defaulting-plugin/DefaultInterference.hs

testsuite/tests/plugins/defaulting-plugin/DefaultInvalid.hs

testsuite/tests/plugins/defaulting-plugin/DefaultLifted.hs

...uite/tests/plugins/defaulting-plugin/DefaultMisaligned.hs

...uite/tests/plugins/defaulting-plugin/DefaultMultiParam.hs