Adds style to external example.

Refs #48.

This mostly works but requires `postcss-import` to be installed in the user's workspace. It doesn't bother me that much right now though, since this is all still on `@build_bazel_rules_nodejs` infra. Once we update to Parcel with `@aspect_rules_js`, this shouldn't be an issue.

examples/external/BUILD.bazel

@@ -3,7 +3,7 @@ load("@aspect_rules_js//npm:defs.bzl", "npm_link_package")
 load("@aspect_rules_ts//ts:defs.bzl", "ts_project")
 load("@bazel_skylib//rules:build_test.bzl", "build_test")
 load("@npm_user//:defs.bzl", "npm_link_all_packages")
-load("@rules_prerender//:index.bzl", "prerender_pages", "web_resources_devserver")
+load("@rules_prerender//:index.bzl", "css_library", "prerender_pages", "web_resources_devserver")
 
 npm_link_all_packages(name = "node_modules")
 
@@ -17,6 +17,7 @@ prerender_pages(
     name = "site",
     src = "site.ts",
     scripts = [":script"],
+    styles = [":styles"],
     source_map = True, # TODO(#48): This doesn't seem to work?
     tsconfig = "//:tsconfig.json",
     lib_deps = ["//:node_modules/rules_prerender"],
@@ -29,6 +30,11 @@ ts_project(
     source_map = True,
 )
 
+css_library(
+    name = "styles",
+    srcs = ["style.css"],
+)
+
 web_resources_devserver(
     name = "devserver",
     resources = ":site",