Add conditional check to prevent redefining existing custom stream source element

[omarluq]

This Pull Request introduces a safeguard in the start.js file to prevent the redefinition of existing custom elements. The issue was observed when legacy JQuery page-refresh events or jest-dom testing caused anycable to reinitiate the start function. This led to an unnecessary re-triggering of the custom stream element definition, even when the element was already present in the window.

The proposed solution involves a conditional check before defining the custom element:

if (customElements.get(tagName) === undefined) {
  customElements.define(tagName, C)
}

This approach is consistent with the convention used in the hotwired turbo library.

[palkan]

Thanks!