Update MeshIO for GeometryBasics refactor

[ffreyer]

(I moved the branch to MeshIO for this one)

Changes for https://github.com/JuliaGeometry/GeometryBasics.jl/pull/219

TODO:

Get things working for (some may already work as is, haven't checked yet)

• [x] 2dm
• [x] gts
• [x] ifs
• [x] msh
• [x] obj
• [x] off
• [x] ply
• [x] stl
• [x] test (locally)