Support launching custom emulators

[DanTup]

See comment at #490 (comment) by @nmfisher and the following comments.

This could be useful for:

• Allowing launching of any third party emulator that Flutter doesn't know how to handle
• Changing the command run when launching a known emulator (by allowing reusing the same ID and overriding)

[DanTup]

Here's an example that overrides an existing emulator to force -gpu host (the ID matches the one returned by flutter emulators, so it gets merged).

Note: You can create your own emulators (with unique IDs), but they must still show up in flutter devices after being launched, otherwise VS Code will sit for 60 seconds and then say the emulator did not launch.

[neiljoseph9]

Where are we supposed to type this in??

[DanTup]

@neiljoseph9 the code above is a VS Code setting. You can open your VS Code User Settings by running the Preferences: Open Settings (JSON) command from the VS Code command palette.