[Feature] Minimum Vertex Distance

### What problem does this feature solve?

in the following graph, you cannot vizualise the line between the nodes. for 2 reasons, the lines between the two are barely visible, and then the lines between prod 1 and prod 3 are hidden by the prod 2 node. in one of the examples i change the curve to illustrate that it also doesnt work.

https://echarts.apache.org/examples/en/editor.html?c=sankey-simple&code=PYBwLglsB2AEC8sDeAoWsDOBTAThLGAXMmurGAJ4hbEDkGAhtANZYW0A0p6ANgxcACuYOtBhZO3WFgC2IABYMMEIiTJkAZsADGg1bQYATAFYNtWaNvZSAvl3V8BwgJJhcDSDFUAGe2UMeDMQA2lLoqOrq0AwyNLC0IDjAhgCMkpH-WODyxN5hsHb5ERmw0bF0ickATOklhllgObB5GYUZxRllcQlJhgDMtRn12bn5NlIAun68ECyqoZFF-egYQjjmFb1p05FgDDgA5lgi8ZWGNTvqAG4MPIJxfb7LsDyzWADKlDxxHSW6OFcLARVClnm1IuD1L91KtBOtumdts89odjptkgNLmQbncHk8Si83p8KN9iNDIv9AdBgcRQQTIWQGeFnrD4ejUoNdvsjicehjOQ4iV8fs90JSgRgQS0Skzsbd7sRHmNJqRxjYANxAA

### What does the proposed API look like?

minimumVertexDistance may also be a good name.

{
source: 'prod1',
target: 'prod2',
value: 30,
lineStyle: {
minimumMidpointDistance: 30 || '20%' //20% of distance between depth 0 and depth 1
},
},