worker source map not copied to output dir

[stevenvachon]

Bug Report

Steps to Reproduce:

1. set up a project with a webpack config using @finos/perspective-webpack-plugin
2. compile

Expected Result:

perspective.wasm.worker.js.map in the output directory.

Actual Result:

My project files excluded, only perspective.wasm.worker.js exists in the output directory.

Environment:

"@finos/perspective-viewer": "0.4.3",
"@finos/perspective-viewer-d3fc": "0.4.3",
"@finos/perspective-viewer-hypergrid": "0.4.3",
"@finos/perspective-webpack-plugin": "0.4.1",

Node.js: 12.16.1
macOS 10.15.3