Add Web Server as an option for device targets

[pavanpodila]

Now that web-server is fully supported with hot reload as well as a web-dev config file, we should include that in the menu item for the run targets. Currently it only shows Chrome and other devices:

...it will be good to see "Web Server" as an option here!

[jwren]

@bkonyi -- this list is populated largely by flutter devices, is this something that will be, or should be, added to the flutter devices tool?

[bkonyi]

Yeah, it's currently hidden by default in the tool for historical reasons. We should probably reconsider making it visible by default, but you can also pass the --show-web-server-device to flutter devices to make it appear.

[jwren]

What is the counter argument for the IDEs (VS Code and IJ/AS plugin) to not just start to pass this flag going forward?

[bkonyi]

The main reason I can think of is that the web-server device doesn't offer full debugging support without the Dart Debug Extension installed, so any functionality that requires access to a Chrome debugger will simply not work (e.g., debugging, object inspection, etc). If the IDE plugins aren't wired up to handle errors from some of these VM service RPCs, we could end up in a bad state or crash the plugins (assuming that's possible).