composer: Stop using lock file

Since we are a library that supports wide range of PHP versions,
we cannot lock into a single set of versions of run-time dependencies.

https://getcomposer.org/doc/02-libraries.md#lock-file

Composer 1.10.0 introduced `lock` option to prevent generation of `composer.lock` file:

https://github.com/composer/composer/releases/tag/1.10.0
https://getcomposer.org/doc/06-config.md#lock

This will basically make `composer install` work the same way as `composer update`.

composer.json

@@ -46,6 +46,9 @@
     "autoload-dev": {
         "psr-4": { "Tests\\Readability\\": "tests/" }
     },
+    "config": {
+        "lock": false
+    },
     "scripts": {
         "fix": "php-cs-fixer fix --verbose --diff",
         "phpstan": "phpstan analyze --memory-limit 512M",