Add Packer.pack and Packer.toArrayBuffer

[joshkel]

To mirror patchDocument's outputType parameter.

See #2920

Quoting from that discussion:

Suggestion:

• Add a Packer.pack or Packer.to method that takes an outputType parameter.
• Change the existing Packer.to... methods to use this new method.
• Optionally, add a Packer.toArrayBuffer method for consistency with the existing Packer.to... methods.

I think I may have explained my suggestion poorly - I was suggesting doing all of the above. This PR is an implementation of that. @dolanmiu, if you'd rather that I just add a toArrayBuffer function instead, I'm happy to go that route - just let me know. Thanks!

Specific notes:

• Keeping the current code's behavior of when errors are thrown versus turned into rejected promises required adding an otherwise redundant async (see eslint-disable-next-line require-await).
• Due to File containing only an abstract typescript class with no implementation is reported as having 0% coverage istanbuljs/istanbuljs#547, the output-type.ts file that I extracted shows as having 0% coverage. I can lower the coverage targets or add the const istanbulFix = ''; workaround from that discussion.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (05fcf6e) to head (9887d7a).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2959   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          286       287    +1     
  Lines         8390      8387    -3     
  Branches      1410      1410           
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- Hits          8390      8387    -3     

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