feat: Allow xhr override globally

[alex-barstow]

Description

It is not currently possible to override or add any custom XHR logic at initialization, since the xhr() method is not exposed on the tech until the source is set (see) and uses a reference to the original videojs.xhr().

Although it is unadvisable in most cases (as mentioned in the docs), it is still useful to have the ability to do it, and the existing beforeRequest() hook is insufficient in many cases.

Specific Changes proposed

• Substitute a custom videojs.Vhs.xhr function for videojsXHR, if it is provided
• This PR does not add any documentation, as I wasn't sure if it is something we want to document

[alex-barstow]

I realized that Function.name is unsupported in IE11, and may not be a reliable source of truth if the code is minified. Might be better to set a flag when overriding: videojs.Vhs.xhr.override = true. which @gkatsev had mentioned. Thoughts?

[gkatsev]

Oh, didn't realize that Function.name wasn't available on IE11 because why would it be available there...
I think it's better to add a flag to the XhrFunction ourselves and if it's present we know it hasn't been overridden.

[gkatsev]

Also, we can do the flag just for IE11 so that we can easily delete it once we drop IE11

[gkatsev]

One thing I tried to do while testing is override it but then call the old xhr method, which caused a whole host of issues. Though, I don't think we need to fix it because this should only ever be used in extreme circumstances and comes with no warranty.
Only thing I can think of is whether we want to maybe log a "I sure hope you know what you're doing" the first time we encounter an overriden xhr method.

The code was as follows:

          const old = videojs.Vhs.xhr;

          videojs.Vhs.xhr = (...args) => {
            console.log('overriden!', args[0].uri);
            return old(...args);
          };