Make @oclif/config a prod dependency

[ersinakinci]

This PR makes @oclif/config a prod dependency rather than a dev dependency. The motivation is to fix monorepo projects whose dependencies are managed by pnpm, since pnpm isolates each monorepo project's dependency's dependencies.

Context: when using yarn or npm, @oclif/config is visible to @oclif/command because it's a prod dependency for oclif (i.e., https://github.com/oclif/oclif/blob/master/package.json#L10) and node_modules has a flat structure. However, with pnpm monorepos, oclif's dependencies and @oclif/command's dependencies are isolated from each other, and so @oclif/config isn't visible to @oclif/command.

I believe that this behavior is not only helpful for those of us using pnpm, it also implements the correct behavior since @oclif/config is called at runtime and should therefore be properly considered a prod dependency.

Fixes #59

[codecov]

Codecov Report

Merging #60 into master will decrease coverage by 0.78%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
- Coverage   68.07%   67.28%   -0.79%     
==========================================
  Files           5        5              
  Lines         166      162       -4     
  Branches       35       35              
==========================================
- Hits          113      109       -4     
  Misses         33       33              
  Partials       20       20

Impacted Files	Coverage Δ
src/util.ts	38.88% <0%> (-5.56%)	⬇️
src/index.ts	77.77% <0%> (+0.5%)	⬆️

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[ersinakinci]

Not sure why code coverage went down...?