Change Mesh block default type to MeshGeneratorMesh

[dschwen]

Reason

Our JSON syntax dump does not adequately describe the valid syntax for validation purposes. Mesh/type defaults to FileMesh in order to support legacy

[Mesh]
  file=foo.e
[] 

blocks, however if MeshGenerator blocks exist below Mesh, the default is internally switched to MeshGeneratorMesh.
This is a bit problematic, as file is listed as a required parameter under Mesh blocks without an explicitly specified type parameter.

Design

It might be time to make the default always be MeshGeneratorMesh and add a deprecated parameter file to MeshGeneratorMesh to warn/error out on the legacy use. If we can figure out a way to make MeshGeneratorMesh even load a file to support that deprecated syntax we could just warn, otherwise it would have to be a hard error (that could affect a lot of Bison files) - but with a proper deprecation message ("Add type=FileMesh to your [Mesh] block") it should be pretty simple for users to fix this.

Impact

A step towards validatable input files.