Is your feature request related to a problem? Please describe.
Currently our CLI uses ugly BSD-style options (long options with a single dash). Using a double dash for long options is more usual in modern programs (this is "GNU style"). Also our names are inconsistent,

• eg -failOnViolation also has the variant --failOnViolation as synonym, but it's the only option that does
• eg we use inconsistent word splitting conventions throughout, eg failOnViolation is camel case, -norulesetcompatibility is "clump-together case" as far as I'm concerned, while -no-cache is in the more usual style for long CLI options

Describe the solution you'd like

• Add long variants for all our CLI options that
  • are GNU style (--)
  • use a consistent casing convention, eg --fail-on-violation, --no-ruleset-compatibility
• Output a deprecation warning when the old variants are used.
• Remove the old variants in PMD 7

Additional thoughts:

• -version should NOT be renamed to --version. As per the gnu standards, the --version option should instead print the version of PMD. I suggest renaming it to --use-version.
• CPD uses GNU style options and mostly uses --this-style but --failOnViolation is camel case and could also be updated
• I'm not sure if we should rename --filelist to --file-list. It feels like a single word now with a specialized meaning, but I'm not sure that's the case