fix: Fixes the dsn behaviour

[HazAT]

This fixes the behaviour to basically support running sentry.io on a sub path like:

https://mysupercooldomain.com/sentry

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@edd08f9). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #1411   +/-   ##
=========================================
  Coverage          ?   84.95%           
=========================================
  Files             ?       33           
  Lines             ?      771           
  Branches          ?      122           
=========================================
  Hits              ?      655           
  Misses            ?      110           
  Partials          ?        6

Impacted Files	Coverage Δ
packages/core/src/dsn.ts	100% <100%> (ø)

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