feat: add MultiReactionBuilder widget

[amondnet]

Merges multiple ReactionBuilder widgets into one widget tree.

MultiReactionBuilder improves the readability and eliminates the need to nest multiple ReactionBuilders.

By using MultiReactionBuilder we can go from:

ReactionBuilder(
  builder: (context) {},
  child: ReactionBuilder(
    builder: (context) {},
    child: ReactionBuilder(
      builder: (context) {},
      child: ChildA(),
    ),
  ),
)

to:

MultiReactionBuilder(
  builders: [
    ReactionBuilder(
      builder: (context) {},
    ),
    ReactionBuilder(
      builder: (context) {},
    ),
    ReactionBuilder(
      builder: (context) {},
    ),
  ],
  child: ChildA(),
)

MultiReactionBuilder converts the ReactionBuilder list into a tree of nested ReactionBuilder widgets.
As a result, the only advantage of using MultiReactionBuilder is improved readability due to the reduction in nesting and boilerplate.

---

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 melo 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

[codecov]

Codecov Report

Merging #917 (5ba18f2) into master (9a97f09) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #917   +/-   ##
=======================================
  Coverage   98.99%   98.99%           
=======================================
  Files          56       57    +1     
  Lines        1987     1990    +3     
=======================================
+ Hits         1967     1970    +3     
  Misses         20       20           

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

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

Files Changed	Coverage Δ
flutter_mobx/lib/src/multi_reaction_builder.dart	100.00% <100.00%> (ø)
flutter_mobx/lib/src/reaction_builder.dart	100.00% <100.00%> (ø)

---

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