Add length menu for datatable to allow configurable page size

[adangel]

Sometimes it would be useful to change the page size (e.g. pmd/pmd#3363 (comment))

This is possible with the datatable, we use: https://datatables.net/reference/option/lengthChange
We probably just don't show the lengthMenu yet, it needs to be added to the dom option via "l": https://datatables.net/reference/option/dom

pmd-regression-tester/resources/js/project-report.js

Line 55 in c98e4a6

dom: 'Pfrtip', // Search Panes, filtering input, processing display element, table, table information summary, pagination control