feat: warn instead of error on unknown .npmrc configs (#9729)

Unknown configuration keys in .npmrc files now emit a warning by
default, restoring pre-npm-12 behavior, instead of throwing. This
reverts the breaking change from 979518d (#9276) for file-based
configs. The new `strict-npmrc` config (default false) opts back into
treating them as a hard error. Unknown CLI flags and abbreviations
continue to error regardless of this setting.