* feat: change vendor package exports
This patch changes top-level exports for the vendor-specific
packages:
- `playwright-chromium`: now exports an object with a single `chromium`
field
- `playwright-wekbit`: now exports an object with a single `webkit`
- `playwright-firefox`: now exports an object with a single `firefox`
Fixes#814
* fix typo
* address comments
## playwright-chromium
- installs chromium
- exposes chromium api from playwright-core
## playwright-firefox
- installs firefox
- exposes firefox api from playwright-core
## playwright-webkit
- installs webkit
- exposes webkit api from playwright-core
## playwright-core
- downloads no browsers
- contains all of the js code
- designed for internal use
## playwright
- downloads all browsers
- exposes the entire api from playwright-core
## github
- downloads all browsers, generates protocol definitions, builds typescript
- exposes "playwright-core" api
