This PR gives a concrete structure to JsPlugin; name, version, main, root, and paths. All existing means of configuration for JS plugins delegates to JsPlugin. Java jars can now provide JS plugins with this change as well.

This is also a _partial_ improvement for deephaven#4817 (JS Plugins development is slow), in that JsPlugin paths is a mean to reduce the scope of files that needs to be copied. This is _not_ a full solution (which may involve routing paths in Jetty that lead directly to the resources as configured via the JS plugins).

For NPM packages, this PR takes a practical approach as opposed to a "perfect" approach; we are requiring that all "real" files be included under the top-level directory as specified by package.json/main. For example, if main is "build/index.js", we'll include everything under "build/". If main is "dist/bundle/index.js", we'll include everything under "dist/".

The inclusion of the matching python structures will follow shortly, either as additional commits to this PR, or as follow-up PR.

Adds testing, fixes deephaven#4893