Add structured JSON output for `changelog`

[abhinavgautam01]

Most query/network commands support --format json, but git pkgs changelog only emits text.

Structured output would make changelog data easier to consume in release tooling, bots and CI reports.

Suggested CLI:

git pkgs changelog lodash -e npm --from 4.17.20 --to 4.17.21 --format json

Suggested JSON shape:

{
  "package": "lodash",
  "ecosystem": "npm",
  "from": "4.17.20",
  "to": "4.17.21",
  "repository": "https://github.com/lodash/lodash",
  "changelog_filename": "CHANGELOG.md",
  "entries": [
    {
      "version": "4.17.21",
      "content": "..."
    }
  ]
}

Text output should remain the default. Invalid formats should return an explicit error, consistent with other formatted commands.