Improving external C++ integration

Introduction

After #1277, the only required use for forward declarations of functions is for using external C++ code. This feature as currently implemented is sub-par for a number of reasons:

1. We need to globally disable the typechecker pass which verifies that forward declarations all eventually have definitions. (--allow-undefined)
2. We generate a C++ declaration for these functions, which means users must use our complicated templates and backward compatibility breaks any time we change our code generation.
3. At least in CmdStan, the external C++ is outside the generated model's namespace and so it needs to have knowledge of what the model's namespace is.

Proposed change:

A new syntax, which looks like

extern "myfile.hpp" real foo(real a);

(note, extern is already a reserved word in Stan).

This syntax solves each of the 3 problems above:

1. The typechecker knows this is for external C++ and not just a normal forward declaration, so it can locally disable that check
2. Similarly, the backend knows this is for external C++ and can generate no bespoke code of its own. The user can provide any C++ which satisfies the call sites, and this should be much more stable between versions of Stan.
3. Finally, we can paste the contents of myfile.hpp directly into the generated C++ (more likely we will just do a C++-level #include), so that this code will live inside the namespace.

Considerations

• If we ever had a non-C++ backend, one could imagine a extern "foo.py" ... style which does the same thing. Using the filename rather than something like extern "C++" is to allow for item 3 above.
• It might be useful to also allow some code before the model namespace, e.g. if the user's C++ needs its own #includes, we may want those outside the namespace. Something like this seems like it could be handled on the command line, rather than in the language.

Previous discussions:

• https://github.com/stan-dev/stanc3/issues/712
• https://github.com/stan-dev/stanc3/pull/1277#issuecomment-1375768211
• https://github.com/stan-dev/design-docs/pull/45#issuecomment-1290780874

Soliciting opinions @bob-carpenter @mitzimorris @rok-cesnovar @nhuurre