Add implementations declaration

Author: devanych

composer.json

@@ -19,14 +19,18 @@
         "docs": "https://httpsoft.org/docs/emitter"
     },
     "require": {
-        "php": "^7.4|^8.0"
+        "php": "^7.4|^8.0",
+        "psr/http-message": "^1.0"
     },
     "require-dev": {
         "httpsoft/http-message": "^1.0",
         "phpunit/phpunit": "^9.3",
         "squizlabs/php_codesniffer": "^3.5",
         "vimeo/psalm": "^3.14"
     },
+    "provide": {
+        "psr/http-message-implementation": "1.0"
+    },
     "autoload": {
         "psr-4": {
             "HttpSoft\\Emitter\\": "src/"