Fix compatibility with PHP 7.3/nightly

As of PHP 7.3, using `continue` to target a `switch` control structure will throw an `E_WARNING`.

Applying `continue` to a `switch` is equivalent to using `break` and more often than not, a `continue` targeting a higher level control structure is actually intended.

To target the higher level control structure, a numeric argument has to be passed to `continue`.

Refs:
* php/php-src#3364
* https://wiki.php.net/rfc/continue_on_switch_deprecation

This change has been recently committed to PHP itself and will break the build against `nightly` for the current `master` already. See: https://travis-ci.org/jrfnl/PHP_CodeSniffer/jobs/401533444 (this branch, but without commits, so equal to the current `master`).

This PR fixes the instances of this in PHPCS.
All these switches are nested in other control structures, but end the switch end the scope closer of the outer control structure, so using `break` in these cases would be equivalent to using `continue 2`.

FYI: The PHPCompatibility standard will include a sniff to detect this in the near future.

Author: jrfnl

src/Files/File.php

@@ -1408,7 +1408,7 @@ public function getMethodParameters($stackPtr)
                 // If it's null, then there must be no parameters for this
                 // method.
                 if ($currVar === null) {
-                    continue;
+                    continue 2;
                 }
 
                 $vars[$paramCount]            = [];

src/Tokenizers/CSS.php

@@ -390,7 +390,7 @@ public function tokenize($string)
 
                     // Needs to be in the format "url(" for it to be a URL.
                     if ($finalTokens[$x]['code'] !== T_OPEN_PARENTHESIS) {
-                        continue;
+                        continue 2;
                     }
 
                     // Make sure the content isn't empty.
@@ -401,7 +401,7 @@ public function tokenize($string)
                     }
 
                     if ($finalTokens[$y]['code'] === T_CLOSE_PARENTHESIS) {
-                        continue;
+                        continue 2;
                     }
 
                     if (PHP_CODESNIFFER_VERBOSITY > 1) {

src/Tokenizers/Tokenizer.php

@@ -743,7 +743,7 @@ private function createTokenMap()
                 }
                 break;
             default:
-                continue;
+                continue 2;
             }//end switch
         }//end for