Summary
-------

Introduce a new method that returns the main method to invoke per JLS of the given presumably initial class or interface.

Problem
-------

With the introduction of JEP 512, Compact Source Files and Instance Main Methods, the determination of an initial class or interface's main method to be invoked is no longer a simple algorithm. There are now many candidate methods.

Solution
--------

Introduce a new method, `getMainClass`, in `java.lang.Class` that returns the `main` method to invoke. This is helpful for when a class has many candidate methods.

This new method does not involve determining if a class or interface is the initial class or interface, initialization of that class or interface, or how the `main` method should be invoked, such as pushing arguments or initializing an object instance.

Specification
-------------

```
    /**
     * Return the {@code main} method to be invoked per JLS {@jls 12.1.4} when
     * this {@code Class} object represents the initial class or interface, or
     * {@code null} if no candidate method exists.
     * <p>
     * A candidate method:
     * <ul>
     * <li>has a single formal parameter of type {@code String[]}, or has no
     * formal parameter</li>
     * <li>has a return type of {@code void}</li>
     * <li>is public, protected or package private</li>
     * </ul>
     * Multiple candidate methods may be present in a class or interface.  In
     * that case, this method returns the one selected per JLS.
     *
     * @apiNote
     * It is the caller's responsibility to determine whether this {@code Class}
     * object represents the initial class or interface, initialize this class
     * or interface before invocation, and find out how to invoke the returned
     * {@code main} method.
     *
     * @return the main method to be invoked, may be {@code null}
     *
     * @jls 8.2 Class Members
     * @jls 8.4 Method Declarations
     * @jls 8.4.2 Method Signature
     * @jls 12.1.4 Invoke a {@code main} method
     * @since 25
     */
```