Add String constants INIT_NAME, CLASS_INIT_NAME, MTD_void for the names and method type of instance and class initializers. They are commonly used by the Classfile API and occurs in a few places in the JDK as well.

Update occurrences of <init> and <clinit> in core libraries API specification to refer to these constants. The occurrences in code elsewhere will be converted separately for there are too many.

See https://mail.openjdk.org/pipermail/classfile-api-dev/2023-March/000233.html