Less_Parser: Tweak parseRuleProperty to match upstream pattern

Our match pattern still seemed to work correctly as-is, but the latest
version from lessjs-2.5.3.js changed during the 2.x release.

It was last changed uptream in commit
less/less.js@16c4f88b7f9e0

I ran `test/bench.php` before and after and did not find a consistent
change either faster or slower. I re-ran the same before/after with
our `>parseNameValue()` commented-out in case that our improvement
made that optimisation harder to observe, but it also made no
consistent difference. Seems the same either way, but let's match
upstream to be safe.

Change-Id: I72c4b370950bc0526d8e5eae2ec43ab1025a2619

lib/Less/Parser.php

@@ -2594,7 +2594,7 @@ private function parseRuleProperty() {
  // @phan-suppress-next-line PhanPluginEmptyStatementWhileLoop
  while ( $this->rulePropertyMatch( '/\\G((?:[\w-]+)|(?:@\{[\w-]+\}))/', $index, $name ) );
 
- if ( ( count( $name ) > 1 ) && $this->rulePropertyMatch( '/\\G\s*((?:\+_|\+)?)\s*:/', $index, $name ) ) {
+ if ( ( count( $name ) > 1 ) && $this->rulePropertyMatch( '/\\G((?:\+_|\+)?)\s*:/', $index, $name ) ) {
  $this->forget();
 
  // at last, we have the complete match now. move forward,