v8: add v8.startupSnapshot utils

[joyeecheung]

This adds several APIs under the v8.startupSnapshot namespace
for specifying hooks into the startup snapshot serialization
and deserialization.

• isBuildingSnapshot()
• addSerializeCallback()
• addDeserializeCallback()
• setDeserializeMainFunction()

Example snapshot entry point that reads a file from disk, compresses it with gzip and makes it
part of the serialized heap so that the deserialized instance can decompress and log the
content of the file after startup:

'use strict';

const fs = require('fs');
const zlib = require('zlib');
const path = require('path');
const assert = require('assert');

const {
  isBuildingSnapshot,
  addSerializeCallback,
  addDeserializeCallback,
  setDeserializeMainFunction
} = require('v8').startupSnapshot;

const filePath = path.resolve(__dirname, '../x1024.txt');
const storage = {};

assert(isBuildingSnapshot());

addSerializeCallback(({ filePath }) => {
  console.error('serializing', filePath);
  storage[filePath] = zlib.gzipSync(fs.readFileSync(filePath));
}, { filePath });

addDeserializeCallback(({ filePath }) => {
  console.error('deserializing', filePath);
  storage[filePath] = zlib.gunzipSync(storage[filePath]);
}, { filePath });

setDeserializeMainFunction(({ filePath }) => {
  console.log(storage[filePath].toString());
}, { filePath });

Fixes: #42617
Refs: #35711

[nodejs-github-bot]

Review requested:

• @nodejs/startup

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/44390/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/44419/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/44481/

[joyeecheung]

Removed the unused isolate which is breaking a build and updated the example a bit to assert isBuildingSnapshot for clarity.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/44488/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/44580/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/44587/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/44631/

[joyeecheung]

Landed in a36a546