Split the "lib.dom.d.ts" definition file apart

<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ, especially the "Common Feature Requests" section: https://github.com/Microsoft/TypeScript/wiki/FAQ --> ## Search Terms <!-- List of keywords you searched for before creating this issue. Write them down here so that others can find this suggestion more easily --> lib.dom.d.ts, split, modular ## Suggestion <!-- A summary of what you'd like to see added or changed --> I'm currently writing a TypeScript definition file for the API of a game, which will be submitted to [DefinitelyTyped](https://github.com/DefinitelyTyped/DefinitelyTyped) soon. The API of the game exposes a `Notification` class to the global scope, which collides with the one in "lib.dom.d.ts" definition file. Not only that, but it also uses jQuery and `XMLHttpRequest`, forcing me to include that definition file. ## Use Cases <!-- What do you want to use this for? What shortcomings exist with current approaches? --> By splitting the "lib.dom.d.ts" definition file apart, user can select only the APIs they need. This change should not break the backward compatibility of old projects, although old versions of TypeScript will not support projects using the modular definition file. ## Examples <!-- Show how this would be used and what the behavior would be --> For example, the "lib.dom.d.ts" definition file could be split up like this: ``` lib.dom.d.ts |-- lib.dom.elements.d.ts |-- lib.dom.notification.d.ts |-- lib.dom.xhr.d.ts `-- ... (other modules) ``` When some parts of the user's definition file collide with the "lib.dom.d.ts" definition file, the user can use the following instead: ```jsonc { "compilerOptions": { "lib": [ "DOM.Elements", "DOM.XHR", ... // (o…