Adds prerender_resources() example.

Closes #28.

This example uses `prerender_resources()` to generate a couple text files and a binary file, then confirms that they were generated as expected. This shows how users can easily generate non-HTML files in their builds.

Author: dgp1130

examples/prerender_resources/BUILD.bazel

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+load("@npm//@bazel/typescript:index.bzl", "ts_library")
+load("//:index.bzl", "prerender_resources")
+load("//tools:jasmine.bzl", "jasmine_node_test")
+
+ts_library(
+    name = "generator",
+    srcs = ["generator.ts"],
+    deps = ["//packages/rules_prerender"],
+)
+
+prerender_resources(
+    name = "prerender",
+    entry_point = "examples/prerender_resources/generator.js",
+    data = [":generator"],
+)
+
+ts_library(
+    name = "generator_test",
+    srcs = ["generator_test.ts"],
+    testonly = True,
+    data = [":prerender"],
+    deps = [
+        "//common:runfiles",
+        "@npm//@types/jasmine",
+        "@npm//@types/node",
+    ],
+)
+
+jasmine_node_test(
+    name = "test",
+    deps = [":generator_test"],
+)

examples/prerender_resources/README.md

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+# Prerender Resources
+
+This example uses `prerender_resources()` to generate files at build time. These
+files are *not* HTML files and are not processed as such (use
+`prerender_pages()` or `prerender_pages_unbundled()` if you want to generate an
+HTML page). `prerender_resources()` can generate any arbitrary files, such as
+simple text, CSV, or JSON. It can also generate binary files like images,
+protocol buffers, or any other useful format.

examples/prerender_resources/generator.ts

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+import { PrerenderResource } from 'rules_prerender';
+
+/** Generates some resources of any file type. No HTML files here! */
+export default function* (): Generator<PrerenderResource, void, void> {
+    // Can prerender text files.
+    yield PrerenderResource.of('/hello.txt', 'Hello, World!');
+    yield PrerenderResource.of('/goodbye.txt', 'Goodbye, World!');
+
+    // Can prerender binary files.
+    yield PrerenderResource.of('/data.bin', new Uint8Array([ 0, 1, 2, 3 ]));
+}

examples/prerender_resources/generator_test.ts

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+import 'jasmine';
+
+import { promises as fs } from 'fs';
+import { resolveRunfile } from 'rules_prerender/common/runfiles';
+
+const prerenderedDir = resolveRunfile(
+        'rules_prerender/examples/prerender_resources/prerender');
+
+describe('Prerender Resources', () => {
+    it('prerenders `hello.txt`', async () => {
+        const content = await fs.readFile(
+                `${prerenderedDir}/hello.txt`, 'utf-8');
+
+        expect(content).toBe('Hello, World!');
+    });
+
+    it('prerenders `goodbye.txt`', async () => {
+        const content = await fs.readFile(
+                `${prerenderedDir}/goodbye.txt`, 'utf-8');
+
+        expect(content).toBe('Goodbye, World!');
+    });
+
+    it('prerenders `data.bin`', async () => {
+        const content = await fs.readFile(`${prerenderedDir}/data.bin`);
+
+        expect(new Uint8Array(content)).toEqual(new Uint8Array([ 0, 1, 2, 3 ]));
+    });
+});