Replace k6StudioState global variable in main process

[cristianoventura]

NOTE: This issue depends on #378 to be completed.

When splitting the main.ts file, we introduced a global object in the main process called k6StudioState. This object contains variables that are used/mutated for multiple IPC handlers and is accessible anywhere in the main process.

Once #378 is complete, we'd like to replace that global variable with a more robust approach that allows the currently global variables to be injected into the handlers. See proposal in #627 (comment)

Example:

interface ProxyServiceEvents {
  'status:change': [ProxyStatus],
  ready: [void]
}

export class ProxyService extends EventEmitter<ProxyServiceEvents> {
  status: ProxyStatus
  
  start() {}
  stop() {}
}

// handlers/index.ts
export function initialize({ browserServer, proxyService }: Services) {
  browserRemote.initialize(browserServer);
  
  // Right now the `browser` handler parts are actually using a global
  // variable because I didn't want to rewrite that part. But this is what
  // it will look like after this PR is merged.
  browser.initialize(browserServer, proxyService)
}