Add the ability to dump type information to a JSON file

[HertzDevil]

If the environment variable CRYSTAL_DUMP_TYPE_INFO is set, at build time the compiler will emit a bunch of type information to a JSON file at that path. The JSON looks something like:

{
    "types": [
        {
            "name": "Regex",
            "id": 46,
            "min_subtype_id": 46,
            "supertype_id": 188,
            "has_inner_pointers": true,
            "size": 8,
            "align": 8,
            "instance_size": 56,
            "instance_align": 8,
            "instance_vars": [
                {
                    "name": "@re",
                    "type_name": "Pointer(LibPCRE2::Code)",
                    "offset": 8,
                    "size": 8
                },
                {
                    "name": "@jit",
                    "type_name": "Bool",
                    "offset": 16,
                    "size": 1
                },
                {
                    "name": "@source",
                    "type_name": "String",
                    "offset": 24,
                    "size": 8
                },
                {
                    "name": "@match_data",
                    "type_name": "Crystal::ThreadLocalValue(Pointer(LibPCRE2::MatchData))",
                    "offset": 32,
                    "size": 16
                },
                {
                    "name": "@options",
                    "type_name": "Regex::Options",
                    "offset": 48,
                    "size": 8
                }
            ]
        }
    ]
}

At the moment this is intended to be an internal tool that supplements the similarly named CRYSTAL_DUMP_TYPE_ID environment variable. I originally made this to generate human-readable reports from GC heap dumps, but there are probably other good uses like enhancing the debugger support scripts.