Ensure PHPStan works properly and fix errors it flagged

Author: dkotter

.distignore

@@ -18,4 +18,5 @@
 /package.json
 /phpcs-compat.xml
 /phpcs.xml
+/phpstan.neon.dist
 /README.md

.gitattributes

@@ -15,4 +15,5 @@
 /package.json export-ignore
 /phpcs-compat.xml export-ignore
 /phpcs.xml export-ignore
+/phpstan.neon.dist export-ignore
 /README.md export-ignore

composer.json

@@ -14,6 +14,7 @@
     "10up/wp-framework": "~1.0.0"
   },
   "require-dev": {
+    "10up/phpcs-composer": "^3.0",
     "szepeviktor/phpstan-wordpress": "^2.0.1",
     "phpstan/extension-installer": "1.4.x-dev",
     "php-stubs/wp-cli-stubs": "dev-master",