Type hints

I'm in the middle of writing the LSP server I mentioned in #268, and in order to provide more useful information (such as field completions), some kind of type system is essential. Right now I'm planning HM-style type inference and external type definitions for modules just like it is in TypeScript's .d.ts files, but allowing users to define types for function signatures and let/const bindings inline would be very useful.

Would it be possible (acceptable) to add optional type hint syntax in Rhai? I am thinking along the lines of Python's type hints, or a much simpler version of TypeScript's type system.

Type hints would only serve the users and static analyzers, and they could be completely stripped when a script is compiled.

I don't have exact fleshed-out proposal for this yet as I will have to experiment more in the LSP project first, but I would like to see something like this supported in Rhai in the future.

No implementation files or tests are identified in the issue. Start by reviewing the LSP server referenced in #268 and experimenting with the proposed HM-style inference and external definitions; the scope is complete only once an agreed optional syntax and its compiler-stripping behavior are specified.