Refactor Task Internal Modules

[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
• Initial implementation completed in https://github.com/ni/nidaqmx-python/pull/534

Why should this Pull Request be merged?

In this PR, remaining task internal modules will be refactored and documentation will be updated:

• [x] _export_signal.py
• [x] _out_stream.py
• [x] _timing.py
• [x] 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
• [x] channels: _channel.py, _ai_channel.py, _ao_channel.py, _ci_channel.py, _co_channel.py, _di_channel.py, _do_channel.py
• [x] triggering: _triggers.py, _arm_start_trigger.py, _handshake_trigger.py, _pause_trigger.py, _reference_trigger.py, _start_trigger.py

Note: Documentation for some of the functions will be updated once the changes in https://github.com/ni/grpc-device-scrapigen/pull/227 are in.

What testing has been done?

In Progress

[charitylxy]

Will be splitting this PR into smaller piece.