export dialog: show relative size increase when embedding scene

Show size increment of the embedded scene during export, as `+22kB’ after the checkbox.

[dwelle]

Sure.

Make sure this applies:

• The helpers for computing the size (well, I say computing the size, but you’ll effectively need to encode and compress it as if doing it for real) will continue to be lazyloaded (via dynamic`import()).
• Don’t initialize/load/compute if the checkbox is not checked. This means the +1kb label won’t show before then, but it will also ensure we don’t load & compute the stuff unnecessarily when no one cares for it.

[Kartik1397]

Can I work on this?