Implementation of async build script escape hatch

[mrgrain]

Using the escape hatch, provide an implementation that spawns a sub process to run an (more or less) arbitrary build script in async mode, so plugins can be used.

Turn the esbuild-plugins example into a ready to use provider.

[mrgrain]

Using worker threads, this can probably be simplified to a module providing the plugins. I think that will be as good as it gets.

[bilalq]

This would be really nice. Using source maps on the first-party CDK Nodejsfunction construct causes them to completely blow up on occasion. Errors go from bad to catastrophic because large node_modules and JSON files can cause sourcemap resolution to make your function crash due to excessive memory usage. ESBuild plugins allow you to exclude these, but the API exposed through the Nodejsfunction interface doesn't let you use plugins. The custom ESBuild provider solution in this lib works, but it's so much ceremony and boilerplate right now to use.