GLTFExporter: add parseAsync

[marcofugaro]

Related issue: #19118 #22031

Description

This PR adds parseAsync to GLTFExporter to make it conform to other loaders/classes.

[donmccurdy]

Added a suggestion to handle at errors where possible. Won't catch everything since parse doesn't have an error handler itself, but should catch most issues.

[marcofugaro]

Thanks for the suggestion @donmccurdy, we should add an onError parameter to parse() and catch the errors in the promise as well.

But this is the job for another PR. Your solution looks good for now.

[mrdoob]

Thanks!

[donmccurdy]

... we should add an onError parameter to parse() and catch the errors in the promise as well.
But this is the job for another PR. Your solution looks good for now.

Agreed! I think we could make that backwards-compatible by detecting when options are given as the 2nd argument instead and just printing a warning.