readme.md example visitor does not compile

[kaby76]

From the readme.md:

https://github.com/mike-lischke/antlr4ng/blob/e1984c4a8ba6cf8ff2e814666db74ff536345e03/ReadMe.md?plain=1#L80-L99

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.