Allow xdebug.file_link_format from php ini to work when xdebug extension is not loaded

```ini_get('xdebug.file_link_format')``` only returns the configuration value if the xdebug extension is actually loaded (ex: ```zend_extension=xdebug.so```). As xdebug slows down even the smallest application tenfold, it's preferable to comment out the zend_extension line in the .ini unless performing debugging or profiling activities. PHP will not return the value of xdebug.file_link_format with ```ini_get()``` unless the extension is loaded; however, ```get_cfg_var()``` *does* work.

So we can provide handy debugging links to the IDE so long as xdebug.file_link_format is set, regardless as to whether xdebug itself is loaded. Left the check with ```ini_get()``` first in case it was overwritten above the .ini level (ie: web server config or ```ini_set()```).

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 
What about rewritting this to:
```php
$this->fileLinkFormat = $fileLinkFormat ?: ini_get('xdebug.file_link_format') ?: get_cfg_var('xdebug.file_link_format');
```