Add `java` to reserved names to escape extensions to `java_`. This may break existing references to gencode for extensions named `java`.

This prevents shadowing of `java.lang` package commonly used in protobuf gencode. Existing extensions named `java` may or may not previously fail to compile depending on if the contents of their .proto result in gencode using `java.lang`. This is needed to fix `java_features.proto` lite gencode since enum gencode uses `java.lang`. Fields named `java` should already be escaped.

*Warning: This may break user code for existing protos with extensions named `java`. References to the extension should be renamed to use `java_` e.g. registry.add(GeneratedClassName.java_)*

PiperOrigin-RevId: 632508249