Consider lowering the severity of UNSUPPORTED_EXTENSION

[emackey]

I recommend lowering the severity of UNSUPPORTED_EXTENSION from "warn" (1) to "info" (2). The glTF format is very extensible, and we shouldn't be warning users not to extend it just because we're unable to validate those extensions.

VSCode 1.55 has increased the default strength and visual noise of warnings. For example:

Across the title bar, glTF models with vendor extensions have their names turn from white to yellow, and a little number appears next to them. This treatment is not given to "info" or "hint", just "error" (red) and "warn" (seen here).

Similar treatment is given to the list of files when the file tree is visible. Files that use vendor extensions are marked in yellow with numbers next to them, while core files can be shown normally in white.

While this app is only one use of the validator, I think it's not unreasonable for any app to be calling out the warnings like this. But vendor extensions should be treated as natural and harmless, not called out as warnings.