fix: reject unexported struct fields at runtime

[thevilledev]

When the type checker cannot determine a concrete struct type at compile time (e.g. ternary with mixed types), field access falls through to dynamic OpFetch at runtime. The Fetch function in vm/runtime and the get function in builtin/lib used FieldByNameFunc which matched unexported fields, then called Interface() on the resulting reflect.Value, causing:

reflect.Value.Interface: cannot return value obtained from unexported field or method

Add IsExported() guards after FieldByNameFunc in both Fetch() and get() so unexported fields are never accessed via reflection. Error output from tests:

str error: cannot fetch str from main.Env (1:23)
 | (true ? v : "string").str
 | ......................^
integer error: cannot fetch integer from main.Env (1:23)
 | (true ? v : "string").integer
 | ......................^

This pattern is part of the allow list in the fuzzing harness, so the issue should clear out once merged.

Relates to #934.