Add "merge layers" operation to `vsvg` and `vsvg-cli`

[abey79]

This operation simply merges all layers to layer 0, if any (ie lmove all 0 in vpype's parlance).

To use with the CLI:

vsvg --layermerge true path/to/file.svg

This PR doesn't include refinement such as:

• copying instead of moving (lcopy)
• moving only some layers
• choosing the target destination

Also, it will need to properly handle path meta data cascading when that is a thing (#4).