feat: add asciinema plugin

[ashikov]

Summary

Description:

Adds the asciinema shortname for https://github.com/ashikov/asdf-asciinema so users can install the plugin without specifying its repository URL after this contribution is merged.

• Tool repo URL: https://github.com/asciinema/asciinema
• Plugin repo URL: https://github.com/ashikov/asdf-asciinema
• Tracking issue: Submit asciinema to the asdf plugin shortname index ashikov/asdf-asciinema#5

Verification

• scripts/test_plugin.bash --file plugins/asciinema — passed
• scripts/test_plugin.bash --diff origin/master HEAD — passed
• scripts/format.bash — passed; no unrelated formatter changes
• scripts/lint.bash — passed with the repository-pinned Prettier, ShellCheck, and shfmt toolchain
• git diff --check — passed
• Plugin CI on the current default branch is green: https://github.com/ashikov/asdf-asciinema/actions/runs/32940917718

Checklist

• Format with scripts/format.bash
• Test locally with scripts/test_plugin.bash --file plugins/asciinema
• Your plugin CI tests are green
  • https://github.com/ashikov/asdf-asciinema/actions/runs/32940917718