fix: observableset and observablemap notify all listeners when one is added with fireimmediately true

[amondnet]

Describe the changes proposed in this Pull Request.

If the PR fixes a specific issue, reference the issue with Fixes #.

Fixes #956

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Pull Request Checklist

• If the changes are being made to code, ensure the version in pubspec.yaml is updated.
• Increment the major/minor/patch/patch-count, depending on the complexity of change
• Add the necessary unit tests to ensure the coverage does not drop
• Update the Changelog to include all changes made in this PR, organized by version
• Run the melos run set_version command from the root directory
• Include the necessary reviewers for the PR
• Update the docs if there are any API changes or additions to functionality

[netlify]

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[codecov]

Codecov Report

Merging #962 (c9e74ad) into main (74e4a89) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #962   +/-   ##
=======================================
  Coverage   98.99%   99.00%           
=======================================
  Files          57       57           
  Lines        1998     2000    +2     
=======================================
+ Hits         1978     1980    +2     
  Misses         20       20           

Flag	Coverage Δ
flutter_mobx	100.00% <ø> (ø)
mobx	98.55% <100.00%> (+<0.01%)	⬆️
mobx_codegen	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...src/api/observable_collections/observable_map.dart	100.00% <100.00%> (ø)
...src/api/observable_collections/observable_set.dart	100.00% <100.00%> (ø)

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