This fixes bug #60097.
Before two global variables CG(heredoc) and CG(heredoc_len) were used to
track the current heredoc label. In order to support nested heredoc
strings the *previous* heredoc label was assigned as the token value of
T_START_HEREDOC and the language_parser.y assigned that to CG(heredoc).
This created a dependency of the lexer on the parser. Thus the
token_get_all() function, which accesses the lexer directly without
also running the parser, was not able to tokenize nested heredoc strings
(and leaked memory). Same applies for the source-code highlighting
functions.
The new approach is to maintain a heredoc_label_stack in the lexer, which
contains all active heredoc labels.
As it is no longer required, T_START_HEREDOC and T_END_HEREDOC now don't
carry a token value anymore.
In order to make the work with zend_ptr_stack in this context more
convenient I added a new function zend_ptr_stack_top(), which retrieves the
top element of the stack (similar to zend_stack_top()).
# __TRAIT__ behaves like __CLASS__ more or less but is constraint to traits.
# Since traits are not types, there are not many valid use cases, and trying
# to use __TRAIT__ to make traits more like classes is discouraged.
