Simplify filtering of validation report files

[javagl]

Right now, the validator can write the validation result as JSON to an output file, using the --reportFile command line argument.

When used programmatically (as a library), it offers a certain filter functionality, like

  const filteredResult = result
    .filter(ValidationIssueFilters.byIncludedSeverities(ValidationIssueSeverity.ERROR));

to only keep validation errors.

But there is no easy way to apply these filters to a report file that has been written: The ValidationResult class is a real class, and cannot be constructed with JSON.parse.

There should be a helper function for this, to create a ValidationResult from a report file, and filter it post-hoc.