Playwright API is parametrized with a `downloadPath` - a path that
is used to download browsers and to look for downloaded browsers.
This patch starts respecting `downloadPath` as part of
`download-browser.js` utility.
## 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
