Set soft error limit default to -1 (unlimited) (#15138)

Fixes #14915 

When there are more than 200 errors to report, mypy "softly" trims the
output without notifying the user that it did so. We want to currently
disable this by setting the `--soft-error-limit` default to `-1` which
would list out _all_ the errors.

mypy/defaults.py

@@ -44,5 +44,5 @@
 ]
 
 # Threshold after which we sometimes filter out most errors to avoid very
-# verbose output
-MANY_ERRORS_THRESHOLD: Final = 200
+# verbose output. The default is to show all errors.
+MANY_ERRORS_THRESHOLD: Final = -1