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)

Start with the linked playground examples for heredoc and nowdoc formatting, comparing the current and envisioned output. Done means the formatter consistently aligns content and closing tags according to whether the opening tag is on its own line or follows code, with coverage for the shown cases.