mlr summary verb, exec() function, mlr cat --filename, multiline string literals, and more

What's Changed

Major:

• mlr summary verb by @johnkerl in #1056
• feat: system/exec() function call ( #1043) by @forbesmyester in #1067 and #1071
• Support simplified sort-map-by-value in the DSL by @johnkerl in #1069
• mlr cat --filename / --filenum by @johnkerl in #1080
• Allow multi-line string literals in the DSL by @johnkerl in #1070

Minor:

• Make PPRINT empty-string markers readable as such by @johnkerl in #1059
• Allow "\n" in mlr repl prompt by @johnkerl in #1058

Bugfixes:

• [Docs] moving --xvright out of the FLATTEN-UNFLATTEN FLAGS section by @trantor in #1065
• Fix doc typo by @luzpaz in #1054
• Fix natsort of empty strings; support mlr sort -rt same as -tr by @johnkerl in #1068

Internal:

• Reduce number of os.Exit callsites, part 1 of n by @johnkerl in #1055
• delete unreachable test code caused by os.Exit by @Abirdcfly in #1073
• Bump github.com/mattn/go-isatty from 0.0.14 to 0.0.16 by @dependabot in #1074

New Contributors

• @luzpaz made their first contribution in #1054
• @Abirdcfly made their first contribution in #1073

Full Changelog: v6.3.0...v6.4.0