Replace webpack and rollup composite build with esbuild, substantially improving the developer experience, as all JavaScript/TypeScript/CSS assets can be built in under 10 seconds, as opposed to 2-3 minutes on webpack. @finos/perspective-webpack-plugin still works and can build these assets via webpack, so this change should really only affect how perspective is built for packaging.

The main potentially-breaking change is that NPM packages have moved from "main"/"module"/"browser"/etc key menagerie to "externals", which is only supported by webpack ^5 and node 12.7.1. For webpack, while support for version ^4 was already deprecated in @finos/perspective-webpack-plugin, perspective could still be built without the plugin using the package's UMD module output (the default); this is no longer the case, though older webpack versions should still be able to link directly to the UMD filepath within the module, e.g. @finos/perspective/dist/umd/perspective.js. Note also that externals, unlike the previous package descriptors, makes other files private by default, so there is some risk of unexpected file resolution issues if a package imports something we did not expect.