minor #15847 [DomCrawler] Optimize the regex used to find namespace prefixes (stof)

This PR was merged into the 2.7 branch.

Discussion
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[DomCrawler] Optimize the regex used to find namespace prefixes

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

I ran Blackfire on the [Mink BrowserKitDriver](https://github.com/minkphp/MinkBrowserKitDriver) testsuite (which is a realistic usage of the Mink API btw, as the driver testsuite is basically a functional test of a fixtures website, where failures are considered to be caused by the driver misbehaving).
I discovered that 17% of the time was spent in ``preg_match_all`` to look for prefixes in the XPath namespace prefixes (Mink does not use any namespace prefix in its Xpath queries).

This optimizes the regex to use a possessive quantifier. This avoids useless backtracking when matching (due to the way the regex is written, backtracking cannot allow finding more matches here). See https://blackfire.io/profiles/compare/21aaebf3-e38f-456a-8fc9-cf7d9e2a35eb/graph for the improvement.
The optimization is applied in the 2.7 branch, because this regex does not exist in 2.3: the registration of namespaces was added in 2.4

Commits
-------

02818e4 [DomCrawler] Optimize the regex used to find namespace prefixes

src/Symfony/Component/DomCrawler/Crawler.php

@@ -1036,13 +1036,13 @@ class Crawler extends \SplObjectStorage
     }
 
     /**
-     * @param $xpath
+     * @param string $xpath
      *
      * @return array
      */
     private function findNamespacePrefixes($xpath)
     {
-        if (preg_match_all('/(?P<prefix>[a-z_][a-z_0-9\-\.]*):[^"\/:]/i', $xpath, $matches)) {
+        if (preg_match_all('/(?P<prefix>[a-z_][a-z_0-9\-\.]*+):[^"\/:]/i', $xpath, $matches)) {
             return array_unique($matches['prefix']);
         }