Revise JSON format

The current JSON format is

{
    "meta": {
        "julia_version": "1.10.1",
        "solver_version": "c6184f4-dirty",
        "solver_name": "TrixiParticles.jl"
    },
    "mid_point_y_solid_1": {
        "n_values": 4,
        "time": [
            0.0,
            0.01,
            0.02,
            0.03,
        ],
        "system_name": "solid",
        "values": [
            0.01,
            0.009900000027477278,
            0.009598224579854263,
            0.009120736097874588,
        ],
        "datatype": "Float64",
        "type": "series"
    },
    "mid_point_x_solid_1": {
        "n_values": 4,
        "time": [
            0.0,
            0.01,
            0.02,
            0.03,
        ],
        "system_name": "solid",
        "values": [
            0.4,
            0.39999999975087647,
            0.3999997685917326,
            0.3999980137707291,
        ],
        "datatype": "Float64",
        "type": "series"
    }
}

For large files, there are a lot of duplicate time vectors. I suggest we extract the time to only have it once. So I guess as data["time"] or so.

Also, wouldn't it be nicer to do data["fluid_1"]["pressure"]["values"] instead of data["pressure_fluid_1"]["values"]?
Then I suggest the following format (open for discussion):

{
    "meta": {
        "julia_version": "1.10.1",
        "solver_version": "c6184f4-dirty",
        "solver_name": "TrixiParticles.jl"
    },
    "time": {
        "n_valus": 4,
        "values": [
            0.0,
            0.01,
            0.02,
            0.03,
        ],
        "datatype": "Float64",
        "type": "series",
    },
    "solid_1": {
        "mid_point_y": {
            "n_values": 4,
            "values": [
                0.01,
                0.009900000027477278,
                0.009598224579854263,
                0.009120736097874588,
            ],
            "datatype": "Float64",
            "type": "series",
        },
        "mid_point_x": {
            "n_values": 4,
            "values": [
                0.4,
                0.39999999975087647,
                0.3999997685917326,
                0.3999980137707291,
            ],
            "datatype": "Float64",
            "type": "series"
        },
    },
}

The issue names no files, tests, or entry points. Locate the Julia code that serializes the current JSON output, then confirm the agreed layout and its consumers; done means producing the revised nested format with one shared time vector.