Added `crossOrigin`, `onLoad`, and `onLoadFailed` config options

[fluidsonic]

• crossOrigin: "anonymous" forces CORS when loading the Messenger script (<script crossOrigin="anonymous" …>). That in turn allows the website/app to receive full error information if the Messenger script throws an error. That's important for error logging and follow-ups with Intercom's own support.
• onLoad and onLoadFailed are called when the Messenger script is loaded successfully or not loaded successfully. That allows for
  • showing a pending/loading animation while the chat is loading
  • showing alternative means of communication in case the Messenger can't be loaded (network issues, Intercom downtime, firewall, browser extensions that block Intercom, etc.)
• README and Cypress tests are updated accordingly

<IntercomProvider
    …
    crossOrigin="anonymous"
    onLoad={() => console.log("Loaded!")}
    onLoadFailed={() => console.log("Failed to load!")}
>

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: e7954c5f5d6c35e724bffe5c168b1057da7a2c60

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ------------------ | ----- | | react-use-intercom | Minor |

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