[GR-46507] Support instance main methods (JEP 445)

[graalvmbot]

With this change, native-image supports running new main variants as proposed by JEP 445 as a preview feature of JDK 21. For example:

$ cat UnnamedHello.java
void main() {
    System.out.println("Hello, World!");
}

$ javac --enable-preview --source 21 UnnamedHello.java
Note: UnnamedHello.java uses preview features of Java SE 21.
Note: Recompile with -Xlint:preview for details.

$ native-image --enable-preview UnnamedHello
...
Finished generating 'unnamedhello' in 17.6s.

$ ./unnamedhello
Hello, World!

This change primarily deals with the "instance main methods" part of JEP 445. There is no full support for "unnamed classes" yet.