"eval" JSON manifestation indentation should match "fmt" default (3 v. 2)

When running the _eval_ subcommand and manifesting JSON, _jsonnet eval_ uses **three spaces** for its indentation step, per the `manifestJSON` function in file _core/vm.cpp_ (specifically, [here for arrays](https://github.com/google/jsonnet/blob/master/core/vm.cpp#L2558) and [here for objects](https://github.com/google/jsonnet/blob/master/core/vm.cpp#L2604)). By contrast, the _fmt_ uses **two spaces** for [its default indentation step](https://github.com/google/jsonnet/blob/master/core/formatter.h#L37).

Why do these two subcommands differ in their indentation? It wouldn't matter as much if either they matched, or the indentation used by the _eval_ subcommand was configurable via command-line flag. I'd prefer that they matched at two spaces per step.