Latest phpStorm IDE complains about missing ext-json in composer.json. Added.

b78baf1e · Carsten Rose · 7e70c493 · b78baf1e
Commit b78baf1e authored Feb 18, 2019 by Carsten Rose
--- a/extension/composer.json
+++ b/extension/composer.json
 {
    "require": {
-        "phpoffice/phpspreadsheet": "^1.3"
+        "phpoffice/phpspreadsheet": "^1.3",
+        "ext-json": "*"
    },
    "require-dev": {
        "phpunit/phpunit": "^6.5"