fix(context): Add file size to context, fixes README_FILE_SMALL

[effigies]

We were getting false positives on README_FILE_SMALL:

ReadmeFileSmall:
  issue:
    code: README_FILE_SMALL
    message: |
      The recommended file '/README' is very small.
      Please consider expanding it with additional information about the dataset.
    level: warning
  selectors:
    - match(path, '^/README')
  checks:
    - size > 150

Turns out size wasn't implemented, but the fix is quick.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.36%. Comparing base (93868ab) to head (9570834).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2030      +/-   ##
==========================================
+ Coverage   85.75%   87.36%   +1.60%     
==========================================
  Files          91      132      +41     
  Lines        3785     6330    +2545     
  Branches     1218     1528     +310     
==========================================
+ Hits         3246     5530    +2284     
- Misses        453      709     +256     
- Partials       86       91       +5     

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[rwblair]

We should double check this in the browser to make sure a size is reported on that api's file object.

[effigies]

Confirmed that this issue doesn't falsely show up in the web browser now. Truncating a README to 149 characters does cause the issue to be raised.