feat(web): web compilation: Debug-mode deadkey naming

Taken from PR reviews on #627:

> Can we go a step further for debug compilation? I'd like to see `this.d_0=0; ... this.d_a=10;` ... in the top part of the file, then reference deadkeys accordingly:
>
> `this.s_deadnums=[{d:this.d_0},...];`
>
>This helps the developer to track their deadkeys much better. Obviously, for the non-debug compile, just use integers as you already are.

It turned out to be more difficult to implement in #627 due to deficiencies in existing code for retrieving deadkey names, but would definitely be a nice feature to include in future versions of the Web transpiler.