Update user/workspace settings from vs code extension

Question

I am trying to update setting.json from my vs code extension and I want to add below code

 "files.associations": {
        "*.app": "auraComponent"
    }

In other words I want to add below key value pair from extension to the users who are going to install my app

So I tried putting the below code in extension.js but it didn't update the settings.

import { ConfigurationTarget, workspace } from 'vscode';

const configuration = workspace.getConfiguration('files.associations');
configuration.update('.app', 'auraComponent', ConfigurationTarget.Global).then(() => {
    // take action here
});

Could someone please suggest if I am using the right approach to update the user or workspace settings and also if the code inside extension.js would be executed automatically or not.

Update

I have added extension.js and included the js code as mentioned by mark in his answer

and update my package.json as below

"main": "./extension",
"activationEvents": [
    "*"
],

If I run my extension on debug mode then it works fine but not working in real time.