fix(typescript): ship .d.ts file in npm package (#6811)

In all my excitement about shipping types, we forgot to export the
actual `d.ts` file that makes it all happen... :(

I didn't pick this up locally because I was testing with `npm link`,
which does a symbolic link and therefore doesn't mirror the list of
files that make it into the published package.

To test this change, I made the change and ran `npm pack` to generate a
tar. I then created a new empty directory and did `npm init -y` followed
by `npm install path/to/puppeteer-7.tgz`.

I then initialised TypeScript, and wrote this:

```js
import puppeteer from 'puppeteer';

async function run() {
  const browser = await puppeteer.launch()
  const page = await browser.newPage()
  await page.goto('https://foo.com')
}
run();
```

As I typed I got autocompletions and if I were to make an error, I'd get
a TypeScript error.

There is follow up work to be done because this unfortunately does not
work if you use `require` rather than ES Modules. @AviVahl suggested a
change in https://github.com/puppeteer/puppeteer/issues/6807 but I
cannot get it to work, so I'd like to do more investigation here. In the
mean time, whatever the contents of `cjs-entry.d.ts`, we should
definitely be exporting it as part of the module, so this PR is still
good to land.

package.json

@@ -41,7 +41,8 @@
     "install.js",
     "typescript-if-required.js",
     "cjs-entry.js",
-    "cjs-entry-core.js"
+    "cjs-entry-core.js",
+    "cjs-entry.d.ts"
   ],
   "author": "The Chromium Authors",
   "license": "Apache-2.0",