payroll-type — US:403b:Employee:Roth — needed separate since taxable

Since Roth contributions are taxable, there are some reports that
need to include these amounts in total salary (i.e., when running a
report that seeks to show total taxable income for an employee).  As
such, we need a `payroll-type` specifically for Roth 403(b)
contributions.

conservancy_beancount/plugin/meta_payroll_type.py

@@ -74,6 +74,7 @@ class SalaryHook(_PayrollTypeHook):
         'CA:PTO:Earned',
         'CA:PTO:Taken',
         'US:403b:Employee',
+        'US:403b:Employee:Roth',
         'US:403b:Match',
         'US:General',
         'US:MA:Disability:PFL',