🐞[bugfix] resource builder without fetcher

[nank1ro]

• The select method of a signal now can take a custom options parameter to customize its behaviour.
• Fixed an invalid assert in the ResourceBuilder widget that happens for resources without a fetcher.

[codecov-commenter]

Codecov Report

Merging #12 (dc92611) into main (43ad836) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #12   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          426       427    +1     
=========================================
+ Hits           426       427    +1     

Impacted Files	Coverage Δ
...ackages/solidart/lib/src/core/readable_signal.dart	100.00% <ø> (ø)
...ter_solidart/lib/src/widgets/resource_builder.dart	100.00% <100.00%> (ø)