Build/Test Tools: Enable starting Xdebug on trigger in local environment.

This aims to facilitate setting up Xdebug locally without editing the `tools/local-env/php-config.ini` file, which is not ignored by Git by default.

Note that this does not enable Xdebug by default, since both `LOCAL_PHP_XDEBUG=true` and `LOCAL_PHP_XDEBUG_MODE=debug` are additionally required for Xdebug to work.

Props SirLouen, jules-colle, johnbillion, pento.
Fixes #49953.

git-svn-id: https://develop.svn.wordpress.org/trunk@60060 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

tools/local-env/php-config.ini

@@ -2,3 +2,4 @@ display_errors = On
 error_reporting = -1
 upload_max_filesize = 1G
 post_max_size = 1G
+xdebug.start_with_request=trigger