Fix for missing zen-observable dependencies

[Esemesek]

• Moved @types/zen-observable dependency to optionalDependencies in apollo-client
• Added zen-observable as a dependency in apollo-client

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• If this was a change that affects the external API used in GitHunt-React, update GitHunt-React and post a link to the PR in the discussion.

[apollo-cla]

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[jbaxleyiii]

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[codecov]

Codecov Report

Merging #2442 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2442   +/-   ##
=======================================
  Coverage   86.04%   86.04%           
=======================================
  Files          36       36           
  Lines        2049     2049           
  Branches      490      490           
=======================================
  Hits         1763     1763           
  Misses        277      277           
  Partials        9        9

---

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[codecov]

Codecov Report

Merging #2442 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2442   +/-   ##
=======================================
  Coverage   86.04%   86.04%           
=======================================
  Files          36       36           
  Lines        2049     2049           
  Branches      490      490           
=======================================
  Hits         1763     1763           
  Misses        277      277           
  Partials        9        9

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7aa8cff...2d3f728. Read the comment docs.