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

• [x] I've run the tests to see all new and existing tests pass
• [x] 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 :white_check_mark:

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

Additional details and impacted files

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

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.