Add annotation for context restrictions (internal only, external only, both)

[adlerjohn]

Add an annotation that indicates that a function (not an ABI method) is supported in particular contexts: either internal only, external only, or both. By default, functions can be used in both external and internal contexts.

If a function call is reachable from an unsupported context, the compiler must emit an error.

[tritao]

Duplicate of https://github.com/FuelLabs/sway/issues/963?

[adlerjohn]

:yes: