internal/driver: Controller cleanup

[nesv]

This pull request that cleans up the LinodeControllerServer (renamed to ControllerServer) in the internal/driver package.

What has changed

This is a brief overview of the changes that have been made in this pull request. Please note that these changes only pertain to the controller server.

• Rename LinodeControllerServer to ControllerServer — the "Linode" prefix is somewhat redundant.
• Organize code so that related things are in the same file.
• Document all exported symbols (constants, variables, types, functions and methods).
• Export some constants for documentation purposes — in some cases, having the constant exported so its documentation is readable can be helpful.
• I have taken most of the "static" errors (ones that do not rely on context-specific information), and moved them into package-level sentinel errors, so they can be reused.

There are no changes in functionality, in this pull request. This pull request is only focused on cleaning up the existing code.

Notes for reviewers

When reviewing this pull request, please go through the branch commit-by-commit!

Attempting to review the whole change set in one go is likely to make things more difficult than they need to be.

General:

• [x] Have you removed all sensitive information, including but not limited to access keys and passwords?
• [x] Have you checked to ensure there aren't other open or closed Pull Requests for the same bug/feature/question?

Pull Request Guidelines:

1. [ ] Does your submission pass tests?
2. [ ] Have you added tests?
3. [ ] Are you addressing a single feature in this PR?
4. [x] Are your commits atomic, addressing one change per commit?
5. [x] Are you following the conventions of the language?
6. [ ] Have you saved your large formatting changes for a different PR, so we can focus on your work?
7. [x] Have you explained your rationale for why this feature is needed?
8. [ ] Have you linked your PR to an open issue

[nesv]

LGTM, thanks for the thorough commit messages and comments! 🙏

You are most welcome!

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I am going to squash out the fixup commits prior to merging.

[nesv]

Squashed out fixup commits, and rebased onto main.