Add property URL in Window interface

[xtuc]

Fix error TS2339: Property 'URL' does not exist on type 'Window'.

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[DanielRosenwasser]

Hey @xtuc thanks for the PR. A few things to note.

1. We don't change lib.d.ts in bin. For anything in the default library, the definition is in one of the files in src/lib. Anything in bin is our output following a build step.
2. I think the file is in dom.generated.d.ts, meaning it's not to be manually changed, though I could be wrong.

@mhegazy and @zhengbli might have some useful input on this.

Additionally, I notice that URL is actually a constructor function. I think the URL interface needs to be amended.

[xtuc]

Ok, sorry I did not notice. I'll let you make the necesary changes because I do not know enough about the language.

[mhegazy]

@zhengbli can you take a look.

[mhegazy]

We need an issue to track this.

[xtuc]

I created the issue: #3753

[mhegazy]

thanks!