Ruby: add Customizations.qll file

ruby/change-notes/2021-12-07-customizations.md

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+lgtm,codescanning
+* A new library, `Customizations.qll`, has been added, which allows for global customizations that affect all queries.

ruby/ql/lib/Customizations.qll

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+/**
+ * Contains customizations to the standard library.
+ *
+ * This module is imported by `ruby.qll`, so any customizations defined here automatically
+ * apply to all queries.
+ *
+ * Typical examples of customizations include adding new subclasses of abstract classes such as
+ * `FileSystemAccess`, or the `Source` and `Sink` classes associated with the security queries
+ * to model frameworks that are not covered by the standard library.
+ */
+
+import ruby

ruby/ql/lib/ruby.qll

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+/**
+ * Provides classes for working with Ruby programs.
+ */
+
+import Customizations
 import codeql.ruby.AST