feat: make plugin event subscribers chainable

[antongolub]

await esbuild.build({
  write: false,
  plugins: [{
    name: 'x', setup(build) {
      build
       .onStart(noop)
       .onEnd(noop)
       .onLoad({ filter: /./ }, noop)
       .onResolve({ filter: /./ }, noop)
       .onDispose(noop)
    }
  }]
})