Function loadScriptIfUndefined

loadScriptIfUndefined(windowAttribute: string, url: string): LoadScriptReturns
Load a script from the given URL if it (window["attribute"]) has not already been loaded.

Usage:
```
import { loadScriptIfUndefined } from "mazey";

loadScriptIfUndefined("xyz", "https://example.com/lib/xyz.min.js")
  .then(() => {
    console.log("xyz is loaded.");
  })
  .catch(err => {
    console.log("Failed to load xyz.", err);
  });
```
Output:
```
xyz is loaded.
```
Parameters
- windowAttribute: string
  The name of the window attribute to check (e.g. jQuery, axios, etc.).
- url: string
  The URL of the script to load.
Returns LoadScriptReturns
A Promise that resolves when the script has been loaded.
- Defined in load.ts:452