feat: Add repo download contents sentinel errors

[stevehipwell]

This PR adds Sentinel errors to the RepositoriesService.DownloadContents & RepositoriesService.DownloadContentsWithMeta functions. This is a follow-up to #4153 that improves the ergonomics. These sentinels are required to correctly handle the different paths in these functions without falling back to writing logic against the error messages which is brittle (and are changing in the next release).

@gmlewis since working on #4153 I've found more issues with the pattern it replaced such as always returning file as the type for a submodule due to the directory loop pattern and a "bug" in the 2022 API version. This combined with a lack of Sentinel errors makes using the current released version painful, would it be possible to get a release out once this PR is merged?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.71%. Comparing base (6c643b8) to head (9cf990f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4192   +/-   ##
=======================================
  Coverage   93.71%   93.71%           
=======================================
  Files         209      209           
  Lines       19770    19772    +2     
=======================================
+ Hits        18527    18529    +2     
  Misses       1046     1046           
  Partials      197      197           

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[gmlewis]

Thank you, @stevehipwell!
LGTM.
Awaiting second LGTM+Approval from any other contributor to this repo before merging.

cc: @alexandear - @zyfy29 - @Not-Dhananjay-Mishra - @munlicode

[Not-Dhananjay-Mishra]

LGTM

[gmlewis]

Thank you, @Not-Dhananjay-Mishra!
Merging and I will follow up with a new release shortly thereafter.

[gmlewis]

@gmlewis since working on #4153 I've found more issues with the pattern it replaced such as always returning file as the type for a submodule due to the directory loop pattern and a "bug" in the 2022 API version. This combined with a lack of Sentinel errors makes using the current released version painful, would it be possible to get a release out once this PR is merged?

@stevehipwell - this is now released here:
https://github.com/google/go-github/releases/tag/v86.0.0