Function loadScriptIfUndefined

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

Usage:
```
loadScriptIfUndefined('xyz', 'https://example.com/lib/jquery.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 index.ts:3814