Define the FPI procedure attribute

We need a way to mark account procedures intended for foreign procedure invocation (FPI).

Before adding an attribute, we should decide:

- Whether it marks only directly called FPI procedures or also procedures reached through callbacks and `dyncall`.
- Whether to use `@fpi` or `@account_procedure(fpi)`.
- Whether the assembler should require it to be used with `@account_procedure`.
- Whether it is documentation-only or affects validation and generated interfaces.
- Whether it could later be used by the kernel to restrict FPI calls.
- How it relates to expiration limits, since not every FPI procedure reads mutable state.

After defining these rules, we can add the attribute to the standards library.