fix: plugin types are wrong in CJS

[JasonWeinzierl]

• Fix types in package.json causing package to masquerade as ESM
• Change the default export to be a more helpful name to avoid import-x/no-rename-default.
• Fix incorrect default export in CJS due to a known issue with unbuild.

[github-actions]

LCOV of commit 78c379d during .github/workflows/ci.yml #10

Summary coverage rate:
  lines......: 95.8% (3153 of 3292 lines)
  functions..: 92.3% (191 of 207 functions)
  branches...: 90.6% (566 of 625 branches)

Files changed coverage rate:
                                         |Lines       |Functions  |Branches    
  Filename                               |Rate     Num|Rate    Num|Rate     Num
  =============================================================================
  scripts/postbuild.ts                   | 0.0%     14| 0.0%     1| 0.0%      1
  src/index.ts                           | 100%     97|    -     0|    -      0