Refactor task.py and internal module (in_stream.py)

[charitylxy]

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

What does this Pull Request accomplish?

This PR is 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 _task_modules

Note to Reviewers: This is the initial stage of implementation to ensure the approach is correct before we continue moving other task subobjects to task package.

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:

• Turned task.py into a package.
• Made in_stream.py into private submodule _in_stream.py and moved to task package.
• Added type hints for in_stream and update documentation.

What testing has been done?

Ran poetry run pytest and all tests passed.

[charitylxy]

Hi @bkeryan, wondering if you have the time to take a look at this PR? I've made the changes you requested.