Fix if-tag parsing when 'and/or' appear inside string literals

[petersgecko]

• I've run the tests with vendor/bin/phpunit
• None of the tests were found failing
• I've seen the coverage report at build/coverage/index.html
• Not a single line left uncovered by tests
• Any coding standards issues were fixed with vendor/bin/php-cs-fixer fix

(i've not yet managed to run coverage. some tests fail because APC missing etc, but they fail on master on my machine)

Problem

TagIf splits conditions on and / or using a regex across the entire
markup. This incorrectly treats and / or tokens inside quoted string
literals as logical operators, e.g.:

{% if course == "art and design" %}

This results in the condition being split into multiple parts and
evaluated incorrectly.

Solution

• Respects quoted string literals (single and double quotes)
• Extracts logical operators only when they appear outside strings
• Preserves existing behaviour for complex logical expressions
• Requires no changes to user templates

dunno, is this even a thing?