Fill in mesh field script output

Fill in some non-geo script output strings (mostly mesh fields).

Here is some sample output:

py: gmsh.model.geo.addPoint(0.9, 0.9, 1, 1.0)
py: gmsh.model.mesh.field.setAsBackgroundMesh(3)
py: gmsh.model.mesh.field.setString(2, "F", "F2 + Sin(z) + Cos(z)")
py: gmsh.model.mesh.field.setNumbers(1, "ExcludedSurfacesList", [1, 2, 3])
py: gmsh.model.mesh.field.remove(3)

About gmsh.model.mesh.field.add, for non-geo scripts, it will segfault after printing the output string. I believe this comes from not actually adding the Field in scriptAddCommand but then attempting to access it afterwards here: https://gitlab.onelab.info/gmsh/gmsh/-/blob/45013d06bf8d7fdc631ad2b3b50a746ba0a3cb4b/Fltk/fieldWindow.cpp#L52. I don't think this can be fixed until scriptAddCommand is adjusted for non-geo files.

I had a general question: when should synchronize calls be emitted? As I understand it there is an implicit synchronize between every geo file command (https://gitlab.onelab.info/gmsh/gmsh/-/blob/45013d06bf8d7fdc631ad2b3b50a746ba0a3cb4b/Geo/scriptStringInterface.cpp#L105-109), but this would be very verbose. However I can't think of another way that guarantees the script will be correct (e.g. if you only synchronize when changing between modules, how do you ensure the final synchronize is correct?).