fix: preserve gradientLength when generating discrete gradient color legend

[hydrosquall]

Motivation

• Fix Support gradientLength for quantile and quantize color scales #9399

Testing

• Check in sandbox: the gradientLength property should be respected for a color legend that uses quantile, quantize, or threshold scales.
• Try changing gradientLength to 300, and the legend should get taller.
• Contrast with the production vega lite editor, changing the gradientLength won't do anything.

{
  "$schema": "https://vega.github.io/schema/vega-lite/v5.json",
  "data": {"url": "data/cars.json"},
  "mark": "rect",
  "encoding": {
    "y": {"field": "Origin", "type": "nominal"},
    "x": {"field": "Cylinders", "type": "ordinal"},
    "color": {
      "aggregate": "mean",
      "field": "Horsepower",
      "scale": {"type": "quantile"},
      "legend": {"gradientLength": 30}
    }
  },
  "config": {"axis": {"grid": true, "tickBand": "extent"}}
}

• See also baseline unit test: f13f69f

Notes

• I temporarily patched the type in vega-lite so this PR can be reviewed. However, we could make the diff in this repo smaller if we instead push a change to the LegendType field inside of vega instead.

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[hydrosquall]

Before I add any tests or open PRs in vega, I want to check if this is the behavior (a third legendType called discrete) we want as a public API, or if this was supposed to be a private/internal API.

The discrete legend type was added by @jheer in vega/vega@ddd2bbf , so I'll bring it up as a separate issue in the vega repo vega/vega#3954 .