Use the go-gpt3 package

[RobotSail]

This PR aims to close #87 by replacing our openai package with the upstream go-gpt3 package.
This way, other people besides us can contribute to and benefit from the go-gpt3 repository, and our pull-requests there will also benefit other users.
We also eliminate some amount of technical debt with this transition, since there is now less to maintain within our own repository.

This change also adds a minor refactor which moves config-related stuff into a separate package underneath pkg/cmd.

[codecov-commenter]

Codecov Report

Merging #97 (013791c) into main (2dd6622) will decrease coverage by 6.86%.
The diff coverage is 65.78%.

@@            Coverage Diff             @@
##             main      #97      +/-   ##
==========================================
- Coverage   73.74%   66.88%   -6.87%     
==========================================
  Files           7        7              
  Lines         598      625      +27     
==========================================
- Hits          441      418      -23     
- Misses        135      188      +53     
+ Partials       22       19       -3     

Impacted Files	Coverage Δ
pkg/cmd/config/config.go	0.00% <ø> (ø)
pkg/filemap/filemap.go	53.88% <0.00%> (-5.80%)	⬇️
pkg/cmd/utils.go	78.76% <72.72%> (-0.61%)	⬇️
pkg/cmd/edit.go	83.01% <100.00%> (+3.47%)	⬆️
pkg/cmd/generate.go	86.25% <100.00%> (+1.01%)	⬆️

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[RobotSail]

We'll probably want PR #73 to merge before this one, and we can pull out all of the refactors that happened with moving config into a sub-package.