New experimental features

• experimental_customMergeAllOf
• experimental_defaultFormStateBehavior. mergeDefaultsIntoFormData

@rjsf/core

• Updated MultiSchemaField to call the onChange handler after setting the new option, fixing #3997 and #4314

@rjsf/utils

• Added experimental_customMergeAllOf option to retrieveSchema() and getDefaultFormState() to allow custom merging of allOf schemas
• Made fields with const property pre-filled and readonly, fixing #2600
• Added mergeDefaultsIntoFormData option to Experimental_DefaultFormStateBehavior type to control how to handle merging of defaults
• Updated mergeDefaultsWithFormData() to add new optional defaultSupercedesUndefined that when true uses the defaults rather than undefined formData, fixing #4322
• Updated getDefaultFormState() to pass true to mergeDefaultsWithFormData for defaultSupercedesUndefined when mergeDefaultsIntoFormData has the value useDefaultIfFormDataUndefined, fixing #4322
• Updated getClosestMatchingOption() to improve the scoring of sub-property objects that are provided over ones that aren't, fixing #3997 and #4314

Dev / docs / playground

• Updated the form-props.md to add documentation for the new experimental_customMergeAllOf props and the experimental_defaultFormStateBehavior.mergeDefaultsIntoFormData option
• Updated the utility-functions.md to add documentation for the new optional defaultSupercedesUndefined parameter and the two missing optional fields on getDefaultFormState()
• Updated the custom-templates.md to add a section header for wrapping BaseInputTemplate
• Updated the playground to add controls for the new mergeDefaultsIntoFormData option
  • In the process, moved the Show Error List component over one column, making it inline radio buttons rather than a select