TypeScript style guide additions

[anton-trunov]

The TypeScript style guide was introduced in #1249 and #1250. Here are some additions/changes it could benefit from.

• The style guide should be linked from README.md and/or the contributing guide.
• The style guide's level one header should have "TypeScript" in it to avoid even brief confusion with Tact style guide we are supposed to come up with at some point.

Do

• does not cover the switch statement; in many cases it can be replaced with makeVisitor from src/utils/tricks.ts, for a usage example see src/prettyPrinter.ts
• simple types should perhaps include bigint as well?
• "Don't use guard types x is T. These are just as in disguise. Might be used in very simple cases, such as ADT boilerplate." -- an example is needed

Don't

• "Don't use optional fields foo?: Bar": needs alternative
• "Don't use get and set. See Proxy above.": needs alternative
• "Avoid exceptions. Exceptions are untyped.": needs alternative