fix: Fix memory leak from user script handler

[dpogue]

Platforms affected

iOS

Motivation and Context

Closes #920.

Description

Ensure that we remove the WKWebView script message handler when the web view plugin is disposed. Otherwise it stays in memory holding references to stuff.

Testing

Tests pass

Checklist

• I've run the tests to see all new and existing tests pass
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.35%. Comparing base (c6fd16f) to head (d1b66ee).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1456   +/-   ##
=======================================
  Coverage   78.35%   78.35%           
=======================================
  Files          16       16           
  Lines        1825     1825           
=======================================
  Hits         1430     1430           
  Misses        395      395           

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[erisu]

LGTM