When changing the organization of custom data layers, backwards
compatibility is easy to address, but forward compatibility is more complex.
This commit adds an option to write files with a legacy mesh format to the
N-panel of the file browser in the "Save As" operator. The option will be used
first by D14077, and won't be committed before that.
It's only added to the "Save As" operator because it's possible to see the
operator properties before confirmation, unlike the regular file save operator.
{F13120897 size=full}
Ref T95965