fix: explicit HTTPRequest.resourceType type defs (#6882)
fix: explicit HTTPRequest.resourceType type defs (#6882)

We can use the new `Lowercase` util in TS4 to avoid duplicating the type
and instead lowercase it.

Note we still need to do the work so callbacks are typed correctly:

```ts
page.on('request', request => {

})
```

Right now `request` is `any`, whereas it should be a
`puppeteer.HTTPRequest`. You can manually set the type for now, and I
will work on adding types for events so that this is done automatically
by the compiler in a future release.

Fixes #6854.
puppeteer/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer,puppeteer/puppeteer