allow user input hv | vh | hvh | vhv

[todo]

antv-spec/src/adaptor/g2plot/toConfig.ts

Lines 89 to 94 in f18179e

	// TODO allow user input hv | vh | hvh | vhv
	configs.config.stepType = 'vh';
	} else if (['hv', 'vh', 'vhv', 'hvh'].includes(spec.layer[0].mark.interpolate)) {
	configs.config.stepType = spec.layer[0].mark.interpolate;
	}
	}

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This issue was generated by todo based on a TODO comment in f18179e when #9 was merged. cc @antvis.