chore: add no-external-code rule to Deno Style Guide lint plugin

[iuioiua]

Towards #6551

[codecov]

Codecov Report

Attention: Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.57%. Comparing base (b5a5fe4) to head (bc6a89c).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
_tools/lint_plugin.ts	86.20%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6562      +/-   ##
==========================================
- Coverage   95.21%   94.57%   -0.65%     
==========================================
  Files         577      579       +2     
  Lines       43381    43724     +343     
  Branches     6487     6492       +5     
==========================================
+ Hits        41307    41351      +44     
- Misses       2034     2332     +298     
- Partials       40       41       +1     

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[kt3k]

LGTM 👍

[iuioiua]

I used ignore lint directives instead of plugin logic so that this plugin can be used for other codebases, as per the original intention of #6551. Either way, we can address this if that ends up happening.

[kt3k]

Isn't this rule specific to std? I guess this rule only makes sense as private rule here