Implement errors getter on form builder state

[deandreamatias]

Connection with issue(s)

Close #1014

Solution description

• Filter fields with errors and convert to map with name and errorText

To Do

• Read contributing guide
• Check the original issue to confirm it is fully satisfied
• Add solution description to help guide reviewers
• Add unit test to verify new or fixed behaviour
• If apply, add documentation to code properties and package readme

[codecov]

Codecov Report

Merging #1234 (16359a6) into main (81c3b97) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1234      +/-   ##
==========================================
+ Coverage   86.03%   86.09%   +0.05%     
==========================================
  Files          20       20              
  Lines         716      719       +3     
==========================================
+ Hits          616      619       +3     
  Misses        100      100              

Impacted Files	Coverage Δ
lib/src/form_builder.dart	77.38% <100.00%> (+0.83%)	⬆️

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