Update Function -> SlackFunction

[srajiang]

Summary

This PR adds a few updates to improve naming in the function middleware implementation.

Updates:

• Renames Function class to SlackFunction to reduce confusion as there's a reserved Function in JS.
• Renames success and error utility arguments to completeSuccess and completeError for better consistency with verb syntax for listener arguments.
• Small changes to Function types

Testing

• Build this branch locally and npm link
• hermes create -t slack-samples/bolt-ts-starter-template -b future
• npm run build && hermes manifest to sanity check the manifest.hermes run -v to start the app
• Test in your workspace that the function runs successfully.

This PR addresses much of the feedback in: #1383 with the exception of adding tests. Future PRs should add this coverage, then that PR should be ready to close out!

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (future@b1de803). Click here to learn what that means.
The diff coverage is n/a.

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