Task migration

[JorgeV92]

continue the migration of components from beman.task into beman.execution.

The main changes are:

• Add the public beman/execution/task.hpp header so users can refer to beman::execution::task.
• Move task-related implementation details into beman::execution::detail as part of the beman.task migration.
  The current branch keeps the task internals directly under beman::execution::detail. I would appreciate feedback on whether this layout is perfered, or whether the task-specific components should instead be grouped under a nested namespace such as beman::execution::detail::task for better modularization.
• Update allocator_support.hpp to better align with the allocator model described in P3980R1.

The task implementation is organized under include/beman/execution/detail/task/ instead of placing all task internals directly in detail/.

Current structure:

include/beman/execution/
├── execution.hpp
├── functional.hpp
├── stop_token.hpp
├── task.hpp
└── detail/
    └── task/
        ├── allocator_of.hpp
        ├── completion.hpp
        ├── error_types_of.hpp
        ├── handle.hpp
        ├── poly.hpp
        ├── promise_env.hpp
        ├── state_rep.hpp
        └── stop_source_of.hpp

all task related functionality will be added under /task.

[coveralls]

coverage: 95.442% (+0.03%) from 95.409% — JorgeV92:task-migration into bemanproject:main