PHP 7.3: New DiscouragedSwitchContinue sniff

[jrfnl]

As of PHP 7.3, using continue to target switch will start throwing an E_WARNING.

. "continue" statements targeting "switch" control flow structures will now
generate a warning. In PHP such "continue" statements are equivalent to
"break", while they behave as "continue 2" in other languages.

Refs:

• https://github.com/php/php-src/blob/04e3523b7d095341f65ed5e71a3cac82fca690e4/UPGRADING#L47-L58
• php/php-src@04e3523
• https://wiki.php.net/rfc/continue_on_switch_deprecation

This PR adds a new sniff to detect this.

Notes:

• This sniff runs into the same difficulties with unscoped control structures as the ForbiddenBreakContinueOutsideLoop sniff.
  For this sniff, I've elected to prevent the false positives which plague the other sniff, though this does mean that the sniff will underreport (false negatives) for continue targeting switch when nested control structures without braces are encountered.
  This underreporting is minimized by examining each case individually. Cases with unscoped control structures will be ignored by this sniff. Cases within the same switch without unscoped control structures will be handled correctly.
• Due to differences in the tokenizer array in the various PHPCS versions, this new sniff will only work on PHPCS 2.3.0+.
  The minimum PHPCS requirement in Composer has been adjusted to reflect this.
  As PHPCS 2.3.0 was released over three years ago, I would expect this not to be an issue.
• Includes one fix for an instance in PHPCompatibility itself which is affected by this PHP 7.3 change (in a copy of an upstream PHPCS method).
• Note: PHPCS 3.x is also affected by this and until the PR I've pulled upstream to fix this has been merged, the build for PHPCompatibility against PHPCS 3.x on PHP nightly will fail.

Fixes #686

As this issue will start breaking build for anyone running unit tests against PHP nightly, I've targeted the 8.2.0 release for this sniff.