allow for overriding sourcemap option

[sterlingwes]

We have a webpack configuration that specifies sourcemap configuration using plugins:

//... plugin array

    isDevServer
      ? new webpack.EvalSourceMapDevToolPlugin({
          columns: false, // equivalent to devtool: cheap-eval-source-map
        })
      : new webpack.SourceMapDevToolPlugin({
          append: false,
          filename: '[file].map',
        }),

Since we don't use the devtool config option we need a way to still have esbuild output sourcemaps