Improve formatting of heredoc/nowdoc

As suggested in https://github.com/prettier/plugin-php/issues/1274#issuecomment-570709723:

I'd say, it would be The Prettier Way™ (and also the prettier way ) to figure out one way to print heredocs and to enforce that.

One process which sounds reasonable to me: If the starting tag goes on its own line, align the content with that. Otherwise indent it one tabWidth further.

Examples of the described rule

Example 1

The heredoc-opening tag goes on its own line.

Input:

some_function(<<<'EOT'
Hello World!
EOT
);

Current output:

some_function(
    <<<'EOT'
Hello World!
EOT
);

Envisioned output:

some_function(
    <<<'EOT'
    Hello World!
    EOT
);

Example 2

The heredoc-opening tag goes on the same line as the preceding code.

Input:

$string = <<<'EOT'
Hello World!
EOT;

Current output:

$string = <<<'EOT'
Hello World!
EOT;

Envisioned output:

$string = <<<'EOT'
    Hello World!
    EOT;

The first example is actually already printed correctly, but the second one isn't (playground)

Beginne mit den verknüpften Playground-Beispielen zur Formatierung von heredoc und nowdoc und vergleiche die aktuelle mit der vorgesehenen Ausgabe. Fertig bedeutet, dass der Formatter Inhalte und schließende Tags konsistent ausrichtet, je nachdem, ob sich das öffnende Tag in einer eigenen Zeile befindet oder auf Code folgt, mit Abdeckung der gezeigten Fälle.