fix(bazel): esbuild ESM bundles should prioritize ESM main field

ESBuild by default with `platform: node` will prioritize `main` and then
check `module` if present. We want it  the other way around as ES
modules are easier to bundle and guaranteed to work.

bazel/esbuild/index.bzl

@@ -29,6 +29,7 @@ def esbuild_esm_bundle(name, **kwargs):
 
     args = dict(
         resolveExtensions = [".mjs", ".js", ".json"],
+        mainFields = ["module", "main"],
         outExtension = {".js": ".mjs"},
         # Workaround for: https://github.com/evanw/esbuild/issues/1921.
         banner = {