Allow chaining of imperative task dependency calls

[zanieb]

Summary

Returns self from imperative task dependency calls so they can be changed

Closes #3695

Changes

• Changes return types of set_upstream, set_downstream, and set_dependencies from None to Task
• Returns self from all the above

Importance

Checklist

This PR:

• adds new tests (if appropriate)
• adds a change file in the changes/ directory (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

[zanieb]

This feels a little clunky but I haven't been a big user of the imperative API.