Change ns-export command to incorporate splatfacto naming

[lanxinger]

Change ns-export command to incorporate "splatfacto" naming convention.

[kerrj]

Thanks for the PR! We're hesitant to call the export option splatfacto since the export is compatible with standard .ply exports. splatfacto is a method, but the export type is still the same data format as others. That said, renaming it from gaussian-splatting to something shorter like splat would be nice to make tab completion more intuitive.

[brentyi]

Something shorter or more specific sounds good to me, but ns-export splat seems confusing because of the common .splat file format (which is different from the .ply we're exporting).