[Lens as Code] Refactor colorByValue schema and transforms

[nickofthyme]

Summary

Simplifies colorByValue schema into an easy explicit range configuration.

// before
const steps = [
  { type: 'from', from: 0, color: '#ff0000' }, // from is only allowed on first step
  { type: 'exact', value: 50, color: '#00ff00' }, // exact is confusing
  { type: 'to', to: 75, color: '#0000ff' }, // to is only allowed on last step
];

// after
const steps = [
  { from: 0, to: 50, color: '#ff0000' }, // from is optional
  { from: 50, to: 75, color: '#00ff00' },
  { from: 75, to: 100, color: '#0000ff' }, // to is optional
];

Changes

• remove to, from and exact step types
• merge colorByValueSchema into single schema
• remove top-level min and max properties
• add single step type with optional to and from
• limit min size of steps to 1
• add validation for step configurations
• update color schema tests

Checklist

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Unit or functional tests were updated or added to match the most common scenarios
• Review the backport guidelines and apply applicable backport:* labels.

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[nickofthyme]

We should forbid empty steps, currently the UI will throw an error, be unresponsive and fill everything with a seemingly random purple color (#872e72).