Use TS assertion signature to avoid type casting

[kubk]

Starting from TS 3.7 it's possible to use so-called "assertion signatures" to narrow TS types after guard statements. It's described here: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-7.html#assertion-functions

It allows to avoid type casting. For some reason assertion signatures are available only for function keyword, that's why I had replace const with function

• This PR only introduces changes to documentation.
  • No
• This PR adds new functionality
  • Is there a related issue?
    • No, it's just a an improvement utilizing latest TS features
  • Does the code style reasonably match the existing code?
    • Yes
  • Are the changes tested (using the existing format, as far as is possible?)
    • Yes
  • Are the changes documented in the readme with a suitable example?
    • Not needed
  • Is the table of contents updated?
    • Not needed
  • Is the index.d.ts file updated, using the same (or appropriately modified) description as in the readme?
    • Not needed

[francisrstokes]

Definitely an improvement, thanks @kubk! 🎉