Add TypeScript code variant to walkthroughs

[maral]

Description
One of the frequent issues here seems to be the lack of TypeScript example code in the documentation (see the issues linked) - it's upon the user to guess them correctly. I happened to struggle through that a bit too and decided to do something about it instead of complaining.

This PR is a draft at the moment - before converting the whole walkthrough, I would like to get a second opinion first, so I know I am going about it right the first time.

Issue
Fixes: #3895, #4290, #4395, #4915, #5612

Context
The approach I've chosen is to use GitBook's Tabs. For each code snippet (except those explicitly TypeScript-only), I added a tabs wrapper. Even if the snippets are the same for TS and JS, for the consistency I added the tabs there.

Potential problems:

• The markdown gets a bit verbose and it's not as nice to manage in the long run.
• TypeScript API might still be changing until the 1.0 version is released.
• I don't know how to preview the docs - I asked about it on Discord, I didn't get any helpful reply.
• As far as I found, changing one tab from JS to TS will not change any other tabs, so folks wanting TypeScript (which I hope is the majority) will end up having to switch manually every time they want to copy the code - not ideal. I created an issue in Gitbook's repo, so we'll see if I get any answer there about this.

Checks

• The new code matches the existing patterns and styles.
• The tests pass with yarn test.
• The linter passes with yarn lint. (Fix errors with yarn fix.)
• The relevant examples still work. (Run examples with yarn start.)

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[dylans]

This looks useful and is appreciated @maral !

I wouldn't worry too much about things changing with respect to TypeScript, that's basically always a risk with TS.

It would be nice if GitBook has a solution for the issue you mentioned.