BREAKING CHANGE: This changes the list of diagnostics that are passed to
a diagnostic handler. If a handler is already filtering by severity
itself then this won't break anything, since the handler's filtering
will become a no-op. But handlers which depend on receiving the full
list of diagnostics may break.
Note that diagnostics are only filtered if the handler's configuration
has the `severity` option set. If `severity` is not set, the handler
still receives the full list of diagnostics.
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