Add chat command ( /test ) to load sample assets

[johnshaughnessy]

This chat command enables loading sample assets by name.

There are three types of assets: objects, scenes, and avatars.

• Objects are loaded with the same options as the /add command.
• The --scene flag changes the active scene.
• The --avatar flag changes the active avatar.

Usage:

/test [--scene] [--avatar] [--resize] [--recenter] [--animate] [--no-menu] <AssetName>

Examples:

Load a sample asset as a media object:

/test Billboard

Change the scene:

/test --scene EnvironmentSettings

Change your avatar:

/test --avatar MorphAudioFeedback

Warning: Typos in the AssetName are not validated. Be careful not to change scenes to an nonexistent asset as doing so can create unrecoverable rooms.

[pattersonbl2]

Sending this PR for a spot check to QA to test functionality on all devices.

[netpro2k]

Since these are hardcoded to a specific set of assets we might just exclude the .glb for slightly more convenient typing