Vega: Handle no height/width with autosize=none (#16696)

This works around the issue described in
vega/vega#1146

With this patch, users will be able to set autosize=none,
while at the same time rely on Kibana to set width/height.
This will only be done when width & height are not set.

Testing code:

```hjson
{
  $schema: https://vega.github.io/schema/vega/v3.0.json
  autosize: none
  padding: {left: 32, bottom: 24, top: 10, right: 6}
  background: "#dbb"
  scales: [
    {
      name: x
      type: band
      range: width
      domain: ["a", "b"]
      paddingOuter: 0.05
      paddingInner: 0.95
    }
    {
      name: y
      type: linear
      range: height
      domain: [0, 100]
    }
  ]
  data: [
    {
      name: edges
      values: [
        {x1: "a", x2: "b", y1: 50, y2: 50, value: 100}
      ]
      transform: [
        {
          type: linkpath
          orient: horizontal
          shape: {signal: "'diagonal'"}
          sourceY: {expr: "scale('y', datum.y1)"}
          sourceX: {expr: "scale('x', datum.x1) + bandwidth('x')"}
          targetY: {expr: "scale('y', datum.y2)"}
          targetX: {expr: "scale('x', datum.x2)"}
        }
        {
          type: formula
          expr: range('y')[0]-scale('y', datum.value)
          as: strokeWidth
        }
      ]
    }
  ]
  axes: [
    {orient: "bottom", scale: "x", zindex: 1}
    {orient: "left", scale: "y", zindex: 1}
  ]
  marks: [
    {
      type: path
      name: edgeMark
      from: {data: "edges"}
      encode: {
        update: {
          strokeWidth: {field: "strokeWidth"}
          path: {field: "path"}
        }
      }
    }
  ]
}
```

src/core_plugins/vega/public/data_model/vega_parser.js

@@ -102,7 +102,17 @@ export class VegaParser {
    * @private
    */
   _calcSizing() {
-    this.useResize = !this.useMap && (this.spec.autosize === 'fit' || this.spec.autosize.type === 'fit');
+    this.useResize = false;
+    if (!this.useMap) {
+      // when useResize is true, vega's canvas size will be set based on the size of the container,
+      // and will be automatically updated on resize events.
+      // We delete width & height if the autosize is set to "fit"
+      // We also set useResize=true in case autosize=none, and width & height are not set
+      const autosize = this.spec.autosize.type || this.spec.autosize;
+      if (autosize === 'fit' || (autosize === 'none' && !this.spec.width && !this.spec.height)) {
+        this.useResize = true;
+      }
+    }
 
     // Padding is not included in the width/height by default
     this.paddingWidth = 0;