syntax: update php.syntax for missed reserved words

[KuzinAndrey]

Helper script:

RESERVED=$(
    wget -q -O- https://www.php.net/manual/en/reserved.keywords.php | \
    grep -Eo " class=\"link\">.*</a>$" | grep -v __ | \
    sed 's/ class="link">\(.*\)<\/a>/\1/g'
)
for K in $RESERVED ; do
  grep -q "keyword whole $K " php.syntax || echo $K
done

Output:

abstract
callable
clone
const
enddeclare
endfor
endforeach
endswitch
final
goto
implements
instanceof
insteadof
interface
namespace
trait
use
xor
yield
yield
from

Adding missing words and sorting lines alphabetically.