Remove restriction on code snippet beginning with a PHP open tag

Problem/Motivation

Remove restriction on code snippet beginning with a PHP open tag. This is no longer necessary as the parser will handle snippets with a mix of inline HTML and PHP code.

Proposed resolution

See patch(es).

Remaining tasks

Before committing, code will be reviewed, tested against D8 core files, and documentation updated.

User interface changes

Not applicable.

API changes

To be completed later.