Include link's index in the React Component key when creating the SVG element

[Ellis-808]

Code to support custom color gradients for multiple links between the same two nodes.
Also prevents the duplicate child key error when 2+ links attach to the same two nodes.

(Screenshots) ResponsiveSankey: Link Color-Gradient Issue

Initial Sankey Chart w/ Multiple Links Between Nodes

Link 1: endColor: "de77ae" // The 1st link's color gradient takes precedence

Link 2: endColor: "#92c5de" // this color is ignored

Link 3: endColor: "bcbddc" // this color is ignored

Link 4: endColor: "97e3d5" // this color is ignored

Sankey Chart w/ Proposed Changes: ca335e6

Code to Generate Bug

import { ResponsiveSankey } from '@nivo/sankey';
import { BasicTooltip } from '@nivo/tooltip';

<ResponsiveSankey enableLinkGradient
    colors={(node) => node.nodeColor}
    linkTooltip={({ link }) => {
        const value = `(${link.linkLabel}) ${link.source.nodeLabel} > ${link.target.nodeLabel}: ${link.value}`;
        return <BasicTooltip enableChip color={link.endColor} value={value} />; 
    }}
    data={{
        nodes: [
            { id: "node1", nodeColor: "#e0e0e0", nodeLabel: "Node 1" },
            { id: "node2", nodeColor: "#e0e0e0", nodeLabel: "Node 2" }
        ],
        links: [
            {
                source: "node1",
                target: "node2",
                value: 5,
                linkLabel: "Link #1",
                startColor: "#e0e0e0",
                endColor: "#de77ae" // The 1st link's color gradient takes precedence
            },
            {
                source: "node1",
                target: "node2",
                value: 8,
                linkLabel: "Link #2",
                startColor: "#e0e0e0",
                endColor: "#92c5de" // this color is ignored
            },
            {
                source: "node1",
                target: "node2",
                value: 3,
                linkLabel: "Link #3",
                startColor: "#e0e0e0",
                endColor: "#bcbddc" // this color is ignored
            },
            {
                source: "node1",
                target: "node2",
                value: 1,
                linkLabel: "Link #4",
                startColor: "#e0e0e0",
                endColor: "#97e3d5" // this color is ignored
            }
        ]
    }}
/>

[codesandbox-ci]

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Latest deployment of this branch, based on commit b773c62:

Sandbox	Source
nivo	Configuration

[plouc]

LGTM