Support node >= v18, update JSDoc, add npm badge

[mbland]

Many of these changes are JSDoc updates. Adds a npm badge to the README in advance. This also removes the technically unnecessary and technically incorrect copyright declaration at the top of LICENSE.txt.

The most important change was adding "node": ">=18.0.0" to the "engines" field of package.json. Doing this required moving the existing test/fixtures/jsdocStub/jsdoc script to jsdoc.js and adding a new jsdoc shell wrapper.

The following explanation for this is from the file comment of the new wrapper script.

---

[This wrapper] is necessary to support older Node versions as package.json engines without removing the "type": "module" specifier. Without this shim, running pnpm test under many older verions caused runJsdoc.test.js and main.test.js to fail on the spawn(jsdocPath) call within runJsdoc().

These older versions couldn't grok that the previous jsdoc stub was really written in ECMAScript Module style without a file extension:

  TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension "" for
  .../jsdoc-cli-wrapper/test/fixtures/jsdocStub/jsdoc. Loading
  extensionless files is not supported inside of "type":"module"
  package.json contexts. The package.json file
  .../jsdoc-cli-wrapper/package.json caused this "type":"module"
  context. Try changing
  .../jsdoc-cli-wrapper/test/fixtures/jsdocStub/jsdoc to have a file
  extension. Note the "bin" field of package.json can point to a file
  with an extension, for example
  {"type":"module","bin":{"jsdoc":"./test/fixtures/jsdocStub/jsdoc.js"}}

I tried adding the --experimental-default-type=module flag to the shebang line of the former jsdoc stub. Only after trying that did I realize that #!/usr/bin/env style shebangs don't support passing command line arguments to the specified interpreter.

Hence moving the original jsdoc stub to jsdoc.js and invoking it via this one-line wrapper.