RFC: https://wiki.php.net/rfc/flexible_heredoc_nowdoc_syntaxes
* The ending label no longer has to be followed by a semicolon or
newline. Any non-label character is fine.
* The ending label may be indented. The indentation will be stripped
from all lines in the heredoc/nowdoc string.
Lexing of heredoc strings performs a scan-ahead to determine the
indentation of the ending label, so that the correct amount of
indentation can be removed when calculting the semantic values for
use by the parser. This makes the implementation quite a bit more
complicated than we would like :/
