refactor: (gatsby-plugin-manifest) misc code and doc updates (#12757)

## Description

Code and Docs updates for `gatsby-plugin-manifest`

- Finished moving to module imports
- Finished migrating off of `bluebird` and onto async/await (order of magnitude speed increase)
- Major doc updates
- Improve logging output for generation

benchmarks/plugin-manifest/README.md

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+# gatsby-plugin-manifest performance tests
+
+- Setup: `yarn`
+- Run: `yarn test`
+
+Benchmarks the current production version of the plugin unless you use `gatsby-dev`.
+
+## To benchmark the current branch:
+
+```sh
+# In the root of the Gatsby repository
+$ yarn run watch --scope=gatsby-plugin-manifest .
+```
+
+```sh
+# In ./benchmarks/plugin-manifest
+# You'll need 'gatsby-dev' installed and configured globally.
+$ gatsby-dev --packages gatsby-plugin-manifest
+```
+
+You may now switch branches using `git checkout` and edit code on the current branch. Changes will be compiled into the local `node_modules` for the benchmark.

benchmarks/plugin-manifest/index.js

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+const { onPostBootstrap } = require(`gatsby-plugin-manifest/gatsby-node`)
+const reporter = require(`gatsby-cli/lib/reporter`)
+const fs = require(`fs-extra`)
+
+//Config for executing onPostBootstrap
+const pluginOptions = {
+  name: `GatsbyJS`,
+  short_name: `GatsbyJS`,
+  start_url: `/`,
+  background_color: `#ffffff`,
+  theme_color: `#663399`,
+  display: `minimal-ui`,
+  icon: `../../www/src/assets/gatsby-icon.png`,
+  cache_busting_mode: `none`,
+}
+
+//Global Variables
+let results = { seconds: [], nanoseconds: [] }
+let sum = [0, 0]
+let rounds = process.env.TOTAL_ROUNDS || 20
+
+//Execute a single test of onPostBootstrap
+async function executeBootstrap() {
+  const timeStart = process.hrtime()
+  await onPostBootstrap({ reporter }, pluginOptions)
+  const timeEnd = process.hrtime(timeStart)
+
+  // console.info(
+  //   "Execution time (hr): %ds %dms",
+  //   timeEnd[0],
+  //   timeEnd[1] / 1000000
+  // )
+
+  results.seconds.push(timeEnd[0])
+  results.nanoseconds.push(timeEnd[1])
+  sum[0] += timeEnd[0]
+  sum[1] += timeEnd[1]
+
+  fs.removeSync(`public/icons`)
+  fs.removeSync(`public/manifest.webmanifest`)
+}
+
+//Loop test to run multiple times and calculate results
+async function runTest() {
+  console.info(`Timing 'onPostBootstrap' running ${rounds} times.`)
+
+  for (let i = 0; i < rounds; i++) {
+    // process.stdout.write(`Round ${i + 1}: `)
+    await executeBootstrap(i)
+  }
+
+  let averageSum = [0, 0]
+  averageSum[0] = sum[0] / rounds
+  averageSum[1] = sum[1] / rounds
+
+  console.info(
+    `\nAverage execution time (hr): %ds %dms`,
+    averageSum[0],
+    averageSum[1] / 1000000
+  )
+
+  console.info(
+    `\nMax execution time (hr): ${Math.max(...results.nanoseconds) / 1000000}ms`
+  )
+  console.info(
+    `\nMin execution time (hr): ${Math.min(...results.nanoseconds) / 1000000}ms`
+  )
+
+  console.info(
+    `\nRange execution time (hr): ${(Math.max(...results.nanoseconds) -
+      Math.min(...results.nanoseconds)) /
+      1000000}ms`
+  )
+}
+
+//execute
+runTest()

benchmarks/plugin-manifest/package.json

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+{
+  "name": "benchmark-gatsby-plugin-manifest",
+  "version": "1.0.0",
+  "description": "Run manifest lots of times",
+  "main": "index.js",
+  "scripts": {
+    "test": "node index.js"
+  },
+  "author": "Alex Moon <alex.jared.moon@gmail.com",
+  "license": "MIT",
+  "dependencies": {
+    "fs-extra": "^7.0.1",
+    "gatsby-plugin-manifest": "^2.0.25"
+  }
+}