json: respect duplicate_field_behavior in std.json.Value.jsonParse

[Techatrix]

This test would previously pass successfully even though it shouldn't:

const std = @import("std");
test {
    const parsed_value = try std.json.parseFromSlice(std.json.Value, std.testing.allocator,
        \\{
        \\  "foo": 1,
        \\  "foo": 2
        \\}
    , .{ .duplicate_field_behavior = .@"error" });
    defer parsed_value.deinit();
    try std.testing.expectEqual(2, parsed_value.value.object.get("foo").?.integer);
}

[thejoshwolfe]

is this necessary?

[Techatrix]

Yes, because of these tests. Duplicate field names are still valid JSON grammer and MUST therefore be successfully parsed according to RFC 8259. See nst/JSONTestSuite#56