chore: merge url options

[james-deepsource]

Description

This PR aims at unifying the development and production WebSocket connection URLs into a single option and other improvements.

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Changes in this PR

• Remove urlForDev and urlForProd options.
• Add url option.
• Specify WebSocket URL via runtime config with websocket key.

// nuxt.config.js
export default {
  // Via Runtime config
  publicRuntimeConfig: {
    websocket: {
      url: process.env.WEBSOCKET_URL
    }
  }
};

• Update docs to reflect the changes.
• Update test suite.
• Update references websocket -> WebSocket.

Checklist before merging

• Tests
• Code and Readme documentation

Note to reviewers (Optional)

As suggested by @shivam-deepsource

[shivam-deepsource]

I've added suggestions for the README as a separate PR to this branch, check it out here

Also, can we add validation for the URL, instead of having a default to an external URL, let's disable the plugin if the URL is not present. We can do it in a separate PR

pending changes

[james-deepsource]

@shivam-deepsource PTAL.