Coding style cleanup: mod/wiki/parser/parser.php

This is about to clean mod/wiki/parser/parser.php and make it style compliant:

• Add the file comments block (+ MOODLE_INTERNAL).
• Whitespace fixes (alignment, tabs, trailing...)
• Rename a couple of (internal) variables to follow the coding style.

Note this does not fix the missing (in all methods) phpdoc blocks. Only phpcs.

We don't touch this file much, but every time we do lots of those little things are reported (for example MDL-67114). I started applying the coding style in that issue but wanted to keep that it clean from noise. So doing it here and now.

For your consideration.