frontend: json: parse negative values
frontend: json: parse negative values

Signed-off-by: Karol Gugala <38af71640571ee4b5093d4f69ffca1ac454f2948@antmicro.com>

antmicro/yosys,antmicro/yosys,YosysHQ/yosys,antmicro/yosys,antmicro/yosys,cliffordwolf/yosys,YosysHQ/yosys,cliffordwolf/yosys,YosysHQ/yosys,antmicro/yosys,YosysHQ/yosys,YosysHQ/yosys,antmicro/yosys,antmicro/yosys,cliffordwolf/yosys,antmicro/yosys,YosysHQ/yosys,YosysHQ/yosys,cliffordwolf/yosys,YosysHQ/yosys,cliffordwolf/yosys,cliffordwolf/yosys,cliffordwolf/yosys,cliffordwolf/yosys,cliffordwolf/yosys