fix: improve disabled decoration field form

[deandreamatias]

Connection with issue(s)

Close #1259

Solution description

• Remove readonly property to enable decoration fields

Screenshots or Videos

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 #1261 (c1fd0ac) into main (96eb532) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1261   +/-   ##
=======================================
  Coverage   85.24%   85.24%           
=======================================
  Files          20       20           
  Lines         705      705           
=======================================
  Hits          601      601           
  Misses        104      104           

Impacted Files	Coverage Δ
lib/src/form_builder_field_decoration.dart	84.61% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more