ignoreErrors with a message regex ages badly

Ignoring an error by matching its message couples the configuration to wording that changes between releases.

parameters:
  ignoreErrors:
    # brittle: the wording changed in 1.2
    - '#Cannot call method [a-zA-Z]+() on mixed#'

    # better: scoped to one path, and reported when unused
    -
      message: '#no value type specified in iterable type#'
      path: src/Legacy/Importer.php
      count: 3

  reportUnmatchedIgnoredErrors: true

reportUnmatchedIgnoredErrors is on by default and worth keeping on, because it turns an obsolete ignore into a build failure rather than a silent hole. A count means fixing two of three occurrences also fails the build, which is the right kind of annoying. A baseline is better than hand-written ignores for existing debt; ignores are for the handful of cases that are genuinely correct and unprovable.