fix: improve slugging

[jdx]

This fixes anchors like oclif/plugin-plugins@2281e86#diff-04c6e90faac2675aa89e2176d2eec7d8R18

Don't judge the way I instantiate classes.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@37e28d9). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master      #89   +/-   ##
=========================================
  Coverage          ?   35.22%           
=========================================
  Files             ?       19           
  Lines             ?      687           
  Branches          ?      118           
=========================================
  Hits              ?      242           
  Misses            ?      416           
  Partials          ?       29

Impacted Files	Coverage Δ
src/commands/readme.ts	76.92% <100%> (ø)

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