[DomCrawler] Fixed an issue with namespace prefix matching being to greedy

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9121
| License       | MIT
| Doc PR        | -

The regexp matching prefixes is naive and matches most of strings followed by a colon: `/(?P<prefix>[a-z_][a-z_0-9\-\.]*):[^"\/]/i`.

It is also incomplete as it does not match all the supported characters (like the unicode ones). Implementing [the actual specification](http://www.w3.org/TR/2004/REC-xml-names11-20040204/#NT-PrefixedName) would mean creating a very complex expression, especially because we have to ignore strings contained in quotes etc.

This would match any XML tag name (names in quotes not ignored):

```php
$ncNameStartChar = 'a-z_\\xC0-\\xD6\\xD8-\\xF6\\xF8-\\x{2FF}\\x{370}-\\x{37D}\\x{37F}-\\x{1FFF}\\x{200C}-\\x{200D}\\x{2070}-\\x{218F}\\x{2C00}-\\x{2FEF}\\x{3001}-\\x{D7FF}\\x{F900}-\\x{FDCF}\\x{FDF0}-\\x{FFFD}\\x{10000}-\\x{EFFFF}';
$ncNameChar = $ncNameStartChar.'.\\-0-9\\xB7\\x{0300}-\\x{036F}\\x{203F}-\\x{2040}';
$ncName = '['.$ncNameStartChar.']['.$ncNameChar.']*';
$regexp = sprintf('/(?P<prefix>%s):%s/iu', $ncName, $ncName);
```

Therefore, I decided NOT to throw an exception if found prefix is actually not a prefix. If a prefix used in xpath doesn't exist we'll still get a warning when running it (`DOMXPath::query(): Undefined namespace prefix`).

The current implementation is simple and will work in most situations. For edge cases, it's still possible to register prefixes manually.

What do you think?