readme.md example visitor does not compile

[kaby76]

From the readme.md:

antlr4ng/ReadMe.md

Lines 80 to 99 in e1984c4

	```typescript
	import { ExpressionVisitor } from "./generated/ExpressionVisitor.js";
	class MyVisitor extends ExpressionVisitor<number> {
	public visitAdd = (ctx: AddContext): number {
	return this.visit(ctx.expression(0)) + this.visit(ctx.expression(1));
	}
	public visitMultiply = (ctx: MultiplyContext): number {
	return this.visit(ctx.expression(0)) * this.visit(ctx.expression(1));
	}
	public visitNumber = (ctx: NumberContext): number {
	return Number.parseInt(ctx.NUMBER().text);
	}
	}
	const visitor = new MyVisitor();
	const result = visitor.visit(tree);
	```

This code doesn't compile, tsc --version "Version 5.6.2". MyVisitor.ts:4:47 - error TS1005: '=>' expected. tree is undefined. *Context are all undefined. .text is used instead of .getText(). Also, visitStart() has to be define otherwise none of the other overrides are called, and "null" is returned.

The code should look something like this.

import { StartContext, ExpressionContext, MultiplyContext, DivideContext, AddContext, SubtractContext, NumberContext } from "./ExpressionParser.js";
import { ExpressionVisitor } from "./ExpressionVisitor.js";

export class MyVisitor extends ExpressionVisitor<number> {
  public visitStart = (ctx: StartContext) => {
    return this.visit(ctx.getChild(0));
  }

  public visitAdd = (ctx: AddContext) => {
    return this.visit(ctx.expression(0)) + this.visit(ctx.expression(1));
  }

  public visitMultiply = (ctx: MultiplyContext) => {
    return this.visit(ctx.expression(0)) * this.visit(ctx.expression(1));
  }

  public visitNumber = (ctx: NumberContext) => {
    return Number.parseInt(ctx.NUMBER().getText());
  }
}

[mike-lischke]

I'll add a test case for that in the unit tests.