fix(types): add drawingArea to RadialLinearScale

[anshukushwaha07]

Closes #12026

The drawingArea property exists in the RadialLinearScale implementation (scale.radialLinear.js) and is used for calculating the radius of radar/polar charts.

However, it was missing from the TypeScript definitions, which caused errors like:

const scale = chart.scales.r;
console.log(scale.drawingArea); 
//  TS error: Property 'drawingArea' does not exist

Changes

• Added readonly drawingArea: number to RadialLinearScale in types/index.d.ts.

import { Chart, RadialLinearScale } from "chart.js";

const chart = new Chart(document.createElement("canvas"), {
  type: "radar",
  data: { labels: ["A"], datasets: [{ data: [10] }] }
});

const scale = chart.scales.r as RadialLinearScale;
console.log(scale.drawingArea); //  typed as number