Create TypeScript declarations from JSDoc

[tinovyatkin]

This PR drops manually created and maintained TypeScript typing in favor of automatically generated from JSDoc.
Run npm run prepublishOnly to inspect new one.
Also drops reference to dom.lib as discussed in node-fetch.

[LinusU]

This will use the globally installed version if there is one, or download the latest version. I think it would be better if we could pin a version by adding typescript to our devDependencies, and then referring to that one here:

Suggested change

	"prepublishOnly": "npx typescript --declaration --emitDeclarationOnly --allowJs index.js"
	"prepublishOnly": "tsc --declaration --emitDeclarationOnly --allowJs index.js"

[tinovyatkin]

I can pin TypeScript version at npx: npx typescript@latest ... if a lower version on a published system is the concern? We need it only before publishing, so, not sure if it better to include and maintain TypeScript as a dependency...

[tinovyatkin]

@LinusU @jimmywarting merging?

[LinusU]

I personally feel that this should be a dev-dependency instead of installed on demand using npx. Since this happens on pre-publish we would be shipping types generated by the latest version of TypeScript, without even reviewing how they look. A newer version of TypeScript could change something, and we wouldn't be able to see that until after the package is published 🤔

Is there any downside to adding typescript to our devDependencies?

[tinovyatkin]

Is there any downside to adding typescript to our devDependencies?

Well, they almost the same as benefits:

1. npm install in CI/on a developer machine will run slower by installing unneeded dependency
2. A newer version of TypeScript could produce better output, but we will be unaware as we pinned an old one.

I don't understand why you will not be able to review output if it will be produced by the same command npm run prepublishOnly?

[xxczaki]

A newer version of TypeScript could produce better output, but we will be unaware as we pinned an old one.

@tinovyatkin Maybe we can add "typescript": "*" to devDependencies so that it will always use the latest version?

[tinovyatkin]

@xxczaki I've added typescript into devDependencies (with pinned version, otherwise it makes no sense at all), just to move this PR forward.

I understand that npx is relatively new command and seems strange sometimes, but it's a standard way of lazy-loading dependencies that are one-off commands (https://medium.com/@maybekatz/introducing-npx-an-npm-package-runner-55f7d4bd282b) and still believe that original proposal was better. Anyway, I'm up for whatever team decides and it's not a big deal.