Cannot dynamically set Toast severity

[slumtrimpet]

Breaking our discussion into a new separate issue as requested from:
#1856

This is my usage of toast:

      if (resp != 'OK') {
        let severity = 'info';
        let summary = 'System Status';
        if (resp.indexOf('ERROR') >= 0) {
          severity = 'error';
          summary = 'Validation Error';
        }
        toast.add({
          severity: severity,
          summary: summary,
          detail: resp,
          life: 3000,
        });
      }

With the current implementation, there's not a way to dynamically set 'severity' as typescript can't evaluate that our local variable is one of the valid options from https://github.com/primefaces/primevue/blob/master/src/components/toast/Toast.d.ts#L6.

VSCode gripes about the severity usage above with: