Add support for Firefox-specific APIs

Feature Request

Add types for browser-specific APIs and make them separate. For example:

// This API is cross-browser compatible, so it can be accessed via `browser`
await browser.tabs.create({ url: "https://github.com" });

// This is Chrome-specific API, which is not accessible on Firefox
await chrome.sidePanel.open({ windowId: 123 });

// This is Firefox-specific API, which is not accessible on Chrome
await firefox.sidebarAction.open();

Right now, since 0.20.0, browser implements only Chrome API types, which makes Firefox-specific API harder to access.

Ideally, if possible, it would be also a good idea to setup some linting for that, in case an extension tries to access browser-specific API without first checking which browser it is currently running on.

Is your feature request related to a bug?

#1651

What are the alternatives?

Bring back webextension-polyfill. That way we can use browser for cross-compatible/Firefox-specific APIs, and chrome for Chrome-specific ones.

Additional context

Chrome and Firefox have quite a big set of incompatible APIs, and it would be nice to have access to them as well

No files or tests are named. Start by locating the current browser and Chrome API type definitions, then trace how they are exposed; done means Firefox-specific APIs such as firefox.sidebarAction have separate typings, with the proposed browser-specific access checks addressed if included in scope.