Upgrade decorator syntax to Typescript 5.0

[puchm]

Typescript 5.0 (currently in beta) will introduce stable well-typed decorators. If experimentalDecorators is enabled, the current version will continue to work. If not, the new decorators require some changes to this library.

For example, this is the error that comes with @jsonObject:

Unable to resolve signature of class decorator when called as an expression.
  The runtime will invoke the decorator with 2 arguments, but the decorator expects 0-1.ts(1238)

Since the new version will replace the experimental syntax in the long term, this should be adjusted.

[leshniak]

Looks like the repo is abandoned again. Any alternatives?