feat: basic pr opening

[TomKristie]

Given an origin branch, repo, owner, an upstream owner, and any PR descriptions, create a PR from that origin branch.
It should:

• invoke the Octokit wrapper (for the GitHub API)
• invoke the Octokit create PR function and open a PR when all the fields are valid for the GitHub V3 API
• re-throw the GitHub V3 API error when the API call fails.

Fixes #19

[codecov]

Codecov Report

Merging #33 into master will increase coverage by 3.71%.
The diff coverage is 76.00%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   58.36%   62.07%   +3.71%     
==========================================
  Files           7        8       +1     
  Lines         281      356      +75     
  Branches       11       13       +2     
==========================================
+ Hits          164      221      +57     
- Misses        117      135      +18     

Impacted Files	Coverage Δ
src/github-handler/index.ts	0.00% <0.00%> (ø)
src/types/index.ts	0.00% <0.00%> (ø)
src/github-handler/pr-handler.ts	100.00% <100.00%> (ø)

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[sofisl]

thank you for this!!