[3.x] Add support for svg images in the asset library

[HolonProduction]

Backport of #70317

Some assets to test svg support:

• https://godotengine.org/asset-library/asset/704
• https://godotengine.org/asset-library/asset/103

Note: the icons of the example assets look diffrent in the asset lib compared to the online version. This is because they are using features that nanosvg does not support (text and css styles). When importing them as svg files they look like they are shown in the editor asset lib so this is not a problem of this PR.

@Calinou mentioned here that svg support would propably not be possible in 3.x. From what I can tell it works the same. Please tell me if I am missing some important detail.

[akien-mga]

Thanks!