Start adding stubs and tests for combined axis, and updating RuleTargetAxis to support it.

internal/mappingrules/mapping_rule_axis_combined.go

@@ -0,0 +1,15 @@
+package mappingrules
+
+import "github.com/holoplot/go-evdev"
+
+type MappingRuleAxisCombined struct {
+	MappingRuleBase
+}
+
+func NewMappingRuleAxisCombined(base MappingRuleBase, inputLower *RuleTargetAxis, inputUpper *RuleTargetAxis, output *RuleTargetAxis) *MappingRuleAxisCombined {
+	return nil
+}
+
+func (rule *MappingRuleAxisCombined) MatchEvent(device Device, event *evdev.InputEvent, mode *string) (*evdev.InputDevice, *evdev.InputEvent) {
+	return nil, nil
+}