Implement parsing of template files with a mixture of PHP and HTML syntax

Problem/Motivation

Implement parsing of template files with a mixture of PHP and HTML syntax. This is enabled by the whitespace preservation changes that precede it.

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.