Add last_line and last_column into outputs of the JSON formatter

[pocke]

Problem

Outputs of the JSON formatter does not include ending location.
For example:

"aa
bb
cc"
foo

Run rubocop test.rb -f json | jq .

{
  "metadata": {
    "rubocop_version": "0.51.0",
    "ruby_engine": "ruby",
    "ruby_version": "2.4.2",
    "ruby_patchlevel": "198",
    "ruby_platform": "x86_64-linux"
  },
  "files": [
    {
      "path": "test.rb",
      "offenses": [
        {
          "severity": "warning",
          "message": "Literal `\"aa\nbb\ncc\"` used in void context.",
          "cop_name": "Lint/Void",
          "corrected": false,
          "location": {
            "line": 1,
            "column": 1,
            "length": 10
          }
        }
      ]
    }
  ],
  "summary": {
    "offense_count": 1,
    "target_file_count": 1,
    "inspected_file_count": 1
  }
}

The output has length field, so we can calculate last_line and last_column with these values, but it is difficult if the offense spans multiple lines.

Solution

Add last_line and last_column fields into outputs of the JSON formatter.
By this change, we can get last_line and last_column without calculation.

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests(rake spec) are passing.
• The new code doesn't generate RuboCop offenses that are checked by rake internal_investigation.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[bbatsov]

The output has length field, so we can calculate last_line and last_column with these values, but it is difficult if the offense spans multiple lines.

It's probably better to name the other related properties start_line and start_column, otherwise it's a bit confusing what's what. Obviously we can just add aliases to the existing names or simply rename those.

[pocke]

It's probably better to name the other related properties start_line and start_column, otherwise it's a bit confusing what's what. Obviously we can just add aliases to the existing names or simply rename those.

Right. Renaming will break compatibility, so I'll add the aliases.