Replace derive4j with a sealed interface and records

[abashev]

Follows #98 and targets its branch, so this diff shows only its own change; once #98 is merged it goes to main.

derive4j generated BreakBehaviours for one type, BreakBehaviour: four cases, two of them with a keepIndentWhenInlined flag. Standard Java does the same job now, and open-java-format no longer runs the derive4j processor.

What changes

• BreakBehaviour is a sealed interface whose cases are records: BreakThisLevel, PreferBreakingLastInnerLevel, InlineSuffix and BreakOnlyIfInnerLevelsThenFitOnOneLine. The factories the 25 call sites used moved from BreakBehaviours onto BreakBehaviour, with the same names.
• Level picks the case with a switch over record patterns instead of the Cases visitor and the caseOf chains, so the compiler checks that every case is handled. The JSON serializer and LevelDelimitedFlatValueDocVisitor switch the same way.
• Each case keeps derive4j's toString, such as preferBreakingLastInnerLevel(true), which the debug visitor prints, and the JSON keeps its shape: {"type": ..., "keepIndentWhenInlined": ...}.
• Level.BreakImpl.preferBreakingLastInnerLevel never read the flag and now takes no parameter. Error Prone's UnusedVariable said so once the method no longer implemented an interface.

Checked

• The 15,747 files of the JDK 21 sources format byte for byte as with Use records instead of Immutables for plain value types #98's head.
• ./gradlew test: 1597 tests in :open-java-format, 10 in the IDEA plugin, 51 in the Gradle plugin.
• With -PnativeImage=true, the native image compiles from this and the Gradle plugin's 60 tests and jdk-bootstrap's 8 pass against it.
• formatDiff through the released plugin, which reshaped the new switches.