minor walk tweak

[VanillaMaster]

core idea was to makevisitors keep their this context, so code below could work

class Crawler {
    importNodes = [];
    ImportDeclaration(node) {
        this.importNodes.push(node);
    }
    
}

const crawler = new Crawler();
walk.simple(ast, crawler);
const { importNodes } = crawler;

alos made base a class instance, so overriding its method is easier

[marijnh]

I'm okay with the calling style / this part.

We have a non-class-based system for building up walkers in place already, so I don't want to merge the other changes, though.