Add support for Deno with an index-deno.ts

[tomaka]

Adds a new index-deno.ts file, similar to index-nodejs.ts and index-browser.ts.

This file gets compiled by TypeScript, which generates dist/mjs/index-deno.js.
The content of dist/mjs can then be published to the deno.land/x registry.

The reason for not publishing directly the content of src on the Deno registry is that Deno requires paths to modules to always end with a .ts extension, while regular TypeScript requires paths to end with either no extension or with .js.
It is therefore impossible to write code that works on both non-Deno TypeScript and Deno TypeScript (unless that code is squashed together within a single module, which I've considered, but this isn't worth the trade-off).

Because the Deno types aren't available on NPM (well, they actually are, but not maintained), I had to copy-paste the types found in the Deno repository.
Considering that we are only using stable APIs, I expect these types to never need to be updated. Fingers crossed that I will not be proven wrong.

We now run deno run ./dist/mjs/index-deno.js in the CI, which should ensure that compatibility never breaks.
I've also added a demo/demo-deno.ts file, which isn't tested by CI but can be run from Deno in order to make sure that everything is working.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[github-actions]

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[tomaka]

Should be merged together with #2533