chore: classify agent

[jimmywarting]

Checklist

• [x] I have ensured my pull request is not behind the main or master branch of the original repository.
• [x] I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• [x] I have written a commit message that passes commitlint linting.
• [x] I have ensured that my code changes pass linting tests.
• [x] I have ensured that my code changes pass unit tests.
• [x] I have described my pull request and the reasons for code changes along with context if necessary.

hoping to have some more modernized codebase with classes and better IDE/autosuggestions. some wishful thinking is also to be able to use private class method/fields later on.

[codecov-commenter]

Codecov Report

Patch coverage: 87.50% and project coverage change: -0.01% :warning:

Comparison is base (cfb7b5e) 94.40% compared to head (fca95a3) 94.39%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1777 +/- ## ========================================== - Coverage 94.40% 94.39% -0.01% ========================================== Files 14 14 Lines 1143 1141 -2 ========================================== - Hits 1079 1077 -2 Misses 64 64 ``` | [Files Changed](https://app.codecov.io/gh/ladjs/superagent/pull/1777?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ladjs) | Coverage Δ | | |---|---|---| | [src/node/agent.js](https://app.codecov.io/gh/ladjs/superagent/pull/1777?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ladjs#diff-c3JjL25vZGUvYWdlbnQuanM=) | `88.88% <84.00%> (-0.70%)` | :arrow_down: | | [src/agent-base.js](https://app.codecov.io/gh/ladjs/superagent/pull/1777?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ladjs#diff-c3JjL2FnZW50LWJhc2UuanM=) | `100.00% <100.00%> (ø)` | |

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[titanism]

awesome work - let us know when ready for review. happy to have you as a collab/maintainer too on all of this

[jimmywarting]

Thanks. I'm ready for a review. I just did a simple PR to see if this is active/maintained and what kind of things you are acceptable of. you can maybe expect more PR to come later on.