fix: Update LegendType typescript definition to support 3rd (discrete) type

[hydrosquall]

Motivation

The source code handles 3 types of legend at runtime

vega/packages/vega-parser/src/parsers/legend.js

Lines 35 to 36 in ddd2bbf

	// resolve legend type (symbol, gradient, or discrete gradient)
	type = legendType(spec, scope.scaleType(scale));

vega/packages/vega-parser/src/parsers/legend.js

Lines 80 to 81 in ddd2bbf

	// discrete gradient legend
	else if (type === Discrete) {

The types definitions only permit 2 types of legend at compile time.

vega/packages/vega-typings/types/spec/legend.d.ts

Line 43 in 8a7f2f2

export type LegendType = 'gradient' | 'symbol';

This mismatch contributed to a bug like vega/vega-lite#9399

Proposal

• We update LegendType to support the third type of legend, discrete (implicitly discrete gradient vs continuous gradient). Note this may also impact 1 or more docstrings .

vega/packages/vega-parser/src/parsers/guides/constants.js

Line 27 in 8a7f2f2

export const Discrete = 'discrete';