npm: update dependencies

[Totktonada]

I think it is good time to perform some maintenance activities. I plan to propose several changes and it is good to renew obsolete things before start.

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Changes in short:

• Applied updates enforced by npm v7+ and TypeScript 4.4+.
• Updated ncc (upstream is changed), node-fetch (security) and run npm update.
• Added a short contributor guide.
• Dropped Ubuntu Xenial (16.04) from CI.

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Full list of changes:

• npm: regenerate package-lock.json for npm v7+
• npm/security: update node-fetch (2.6.1 -> 2.6.7)
• npm: update @zeit/ncc@0.22.3 to @vercel/ncc@0.33.1
• npm: update dependencies to latest compatible ones
• build: validate that an error is of type Error
• build: regenerate index.js
• doc: add hints for contributors
• gitignore: add Vim Swap files
• ci: drop ubuntu-16.04

Consider commit messages for details.

[Totktonada]

@rosik, please, take a look if time permits.

[Totktonada]

Yaroslav said: 👍.