RFR: 8304139: Add <init>, <clinit>, and main method constants to ClassDescs

[liach]

Add String constants INIT_NAME, CLASS_INIT_NAME, MTD_void for the names and method type of instance and class initializers; add String constant MAIN_NAME and MTD_void_StringArray for the name and method type of main method.

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

-------------

Commit messages:
 - Add constants for initializers and main

Changes: https://git.openjdk.org/jdk/pull/13020/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13020&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304139
  Stats: 85 lines in 7 files changed: 64 ins; 0 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/13020.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/13020/head:pull/13020

PR: https://git.openjdk.org/jdk/pull/13020