Implement a class for the Profiler API in dwds

The Chrome DevTools protocol has a [`Profiler` API](https://chromedevtools.github.io/devtools-protocol/tot/Profiler), which adds a host of helpful methods for analyzing code. In our case in particular, we would like to make use of `Profiler.takePreciseCoverage` and `Profiler.startPreciseCoverage` to generate code coverage reports.

Overall I think this would look very similar to the [Sources](https://github.com/dart-lang/webdev/blob/master/dwds/lib/src/debugging/sources.dart) class but even simpler. A basic implementation of this API could be as simple as this:

```Dart
class Profiler {
bool _profilerEnabled = false;
final RemoteDebugger _remoteDebugger;

Profiler(this._remoteDebugger);

Future startProfiler() async {
if (_profilerEnabled) {
return;
}
_profilerEnabled = true;
await _remoteDebugger.sendCommand('Profiler.enable');
}

Future startPreciseCoverage() async {
await startProfiler();
return _remoteDebugger.sendCommand('Profiler.startPreciseCoverage');
}

Future takePreciseCoverage() async {
await startProfiler();
return _remoteDebugger.sendCommand('Profiler.takePreciseCoverage');
}
}
```

Obviously there's a million other things you could add and use, but really this is all we'd need to help get the information we want.

Start by reading dwds/lib/src/debugging/sources.dart and the RemoteDebugger usage it demonstrates. Add a similarly structured, simpler Profiler class that enables the Profiler domain and exposes precise-coverage commands. Done means the class sends the requested protocol commands without enabling the profiler more than once.

Written by the indexing model from the issue text.