feat: introduce GitHub release functionality

[bcoe]

Introducing "GitHub Release" functionality, which (similarly to autorelease), will:

• detect a PR tagged with autorelease: pending.
• extract the most recent CHANGELOG entry.
• create a GitHub release with appropriate version # and release notes.

This workflow is for repos that don't use the autorelease tool; @ofrobots you said that if we can get as far as creating a GitHub release, your CI/CD will take care of the npm publication correct? I have your workflow in mind with this branch.

[codecov]

Codecov Report

Merging #85 into master will decrease coverage by 31.74%.
The diff coverage is 40.38%.

@@             Coverage Diff             @@
##           master      #85       +/-   ##
===========================================
- Coverage   82.42%   50.68%   -31.75%     
===========================================
  Files           5        7        +2     
  Lines         330      807      +477     
  Branches       26       28        +2     
===========================================
+ Hits          272      409      +137     
- Misses         56      396      +340     
  Partials        2        2

Impacted Files	Coverage Δ
src/github.ts	22.68% <20.89%> (ø)
src/github-release.ts	55.05% <55.05%> (ø)

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