Merge pull request #2330 from WordPress/feature/composer-make-extension-deps-explicit

Composer: make PHP extension dependencies explicit

Author: dingo-d

composer.json

@@ -18,6 +18,9 @@
 	"require": {
 		"php": ">=5.4",
 		"ext-filter": "*",
+		"ext-libxml": "*",
+		"ext-tokenizer": "*",
+		"ext-xmlreader": "*",
 		"squizlabs/php_codesniffer": "^3.7.2",
 		"phpcsstandards/phpcsutils": "^1.0.8",
 		"phpcsstandards/phpcsextra": "^1.1.0"
@@ -30,6 +33,7 @@
 		"php-parallel-lint/php-console-highlighter": "^1.0.0"
 	},
 	"suggest": {
+		"ext-iconv": "For improved results",
 		"ext-mbstring": "For improved results"
 	},
 	"config": {