Enable Schematron validation, at user option

[AndrewSales]

Should take place before:

https://github.com/AndrewSales/style-check/blob/main/lib/style-check.xqm#L95

• consider making this also a batch transform running a compiled stylesheet (SEF), for performance reasons
• use the current result of sc:validate()/errors/@systemId as the manifest.

[AndrewSales]

halt-on-invalid not respected, see https://github.com/AndrewSales/style-check/blob/main/test/tests.xqm#L319-L320

[AndrewSales]

Implement using modes: current impl to remain in default mode, but halt-on-invalid otherwise.