ObjectLoader: Add async API.

[Mugen87]

Related issue: see #22026.

Description

This PR adds parseAsync(), loadAsync() and parseImagesAsync() to ObjectLoader. This resolves a hack during the image loading process which also closes #13177.

The previous usage of the loader still works. At some point in the future, we might want to add a deprecation message two load() and parse() and then remove both methods.

[Mugen87]

This PR also makes #22024 redundant.

[mrdoob]

Thanks!

[tingspain]

Hello, I think the new changes broke the example of the lightmap. It stoped working. Now, It gives the following uncaught syntax error:
Uncaught SyntaxError: Unexpected reserved word @ webgl_materials_lightmap.html:59

[Mugen87]

It works with Chrome 91.0.4472.114 and Firefox 90 without issues. Maybe you are using an outdated browser?

[tingspain]

Indeed, I was using Opera 73.0.3856.344 and Safari 14.1.1 (16611.2.7.1.4), and on both of them, the example was not working. I tried with chrome 91.0.4472.114, and it worked perfectly. I updated to Opera 77.0.4054.254, and it works perfectly too.

So... thanks for the hint!!