This changes the root object from RemoteBrowser to Playwright,
similar to local driver connection. This way, any remote connection
gets a Playwright object.
This also starts reusing PlaywrightServer class, and introduces
`cli run-server` hidden command that runs ws server on the
specified port.
Previous structure:
```
RemoteBrowser
- browser (using ConnectedBrowser for remote-specific behavior)
- selectors (special instance for this remote connection)
```
New structure:
```
Playwright
- ...
- selectors (special instance for this remote connection)
- preLaunchedBrowser (using ConnectedBrowser for remote-specific behavior)
```
This is a large rework of selectors:
- Each BrowserContext now has a separate Selectors instance that has its own registrations.
Most of them share a single sharedSelectors instance, but contexts created for a connected
browser have their own instance.
- Connected browser now gets a RemoteBrowser object that encapsulates Selectors and Browser.
This Selectors object is registered with the api selectors.
- Public selectors.register api iterates over all registered Selectors channels
and registers in each of them.
- createSelector testing method migrated to ElementHandle._createSelectorForTest.
