fetch typings: support global agumentation

[mfulton26]

Some types are currently defined using class but, from what I can tell, TypeScript classes cannot be globally augmented (see How can I augment a globally declared class?).

e.g.

deno/ext/fetch/lib.deno_fetch.d.ts

Line 413 in 215528f

declare class Response implements Body {

I'd like to see Headers, Request, Response and any other applicable types follow the same pattern as many built-in type definitions:

• interface {{Type}} { … }
• interface {{Type}}Constructor { … }
• var {{Type}}: {{Type}}Constructor;

e.g.

• interface Map { … }
• interface MapConstructor { … }
• var Map: MapConstructor;

This will allow declaration merging for adding missing types or even defining custom ones.

[mfulton26]

I'm not fully aware of how Deno pulls in generated types from TypeScript itself but this might be somewhat dependent on microsoft/TypeScript-DOM-lib-generator#1518.

[sibbng]

Fixed in #19438