feat: add keepAlive to Computed

[amondnet]

Describe the changes proposed in this Pull Request.

This avoids suspending computed values when they are not being observed by anything (see the above explanation). Can potentially create memory leaks.

Fixes #975

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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

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (00fe8b0) 99.00% compared to head (4f3a787) 99.01%.

❗ Current head 4f3a787 differs from pull request most recent head e3a7bc6. Consider uploading reports for the commit e3a7bc6 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #976   +/-   ##
=======================================
  Coverage   99.00%   99.01%           
=======================================
  Files          57       57           
  Lines        2017     2024    +7     
=======================================
+ Hits         1997     2004    +7     
  Misses         20       20           

Flag	Coverage Δ
flutter_mobx	100.00% <ø> (ø)
mobx	98.56% <100.00%> (+<0.01%)	⬆️
mobx_codegen	100.00% <100.00%> (ø)
mobx_lint	∅ <ø> (∅)

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

Files	Coverage Δ
mobx/lib/src/api/annotations.dart	80.00% <100.00%> (ø)
mobx/lib/src/core/computed.dart	100.00% <100.00%> (ø)
mobx_codegen/lib/src/store_class_visitor.dart	100.00% <100.00%> (ø)
mobx_codegen/lib/src/template/computed.dart	100.00% <100.00%> (ø)

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[pavanpodila]

Pls resolve the conflicts as we merged the previous PR that bumped the version

[pavanpodila]

looks good overall..need a version bump...also pls check if there is any drop in test coverage

[amondnet]

@pavanpodila I rebased to the main branch and resolved the conflict.