Refactor Task Internal Modules: Collections

[charitylxy]

• [x] This contribution adheres to CONTRIBUTING.md.

What does this Pull Request accomplish?

This PR aims to address issue brought up in https://github.com/ni/nidaqmx-python/issues/461:

• Currently, task subobject types are defined in internal modules contained in private nidaqmx._task_modules package.
• We want to turn task.py into a package and merge it with the internal modules contained in nidaqmx._task_modules

Refactoring work will be split into 5 PRs:

• [x] Refactoring _task.py and _in_stream.py - https://github.com/ni/nidaqmx-python/pull/534
• [x] Refactoring _export_signal.py, _out_stream.py, _timing.py - https://github.com/ni/nidaqmx-python/pull/539
• [x] Refactoring collections: _channel_collection.py, _ai_channel_collection.py, _ao_channel_collection.py, _ci_channel_collection.py, _co_channel_collection.py, _di_channel_collection.py, _do_channel_collection.py
• [ ] Refactoring channels: _channel.py, _ai_channel.py, _ao_channel.py, _ci_channel.py, _co_channel.py, _di_channel.py, _do_channel.py
• [ ] Refactoring triggering: _triggers.py, _arm_start_trigger.py, _handshake_trigger.py, _pause_trigger.py, _reference_trigger.py, _start_trigger.py

Why should this Pull Request be merged?

In this PR to address https://github.com/ni/nidaqmx-python/issues/461, following changes have been made:

• Moved collections to task package and made its modules private.
• Added type hints for xx_channels in _task.py.
• Documentation updated.

What testing has been done?

Ran poetry run pytest and all tests passed.