feat: introduce value inheritance for custom fields

Author: codescape

src/main/java/de/codescape/jira/plugins/multiplesubtasks/model/Variables.java

@@ -0,0 +1,21 @@
+package de.codescape.jira.plugins.multiplesubtasks.model;
+
+import java.util.regex.Pattern;
+
+/**
+ * Variables are introduced by an `@` character and their value is surrounded by curly braces.
+ */
+public class Variables {
+
+    /**
+     * Pattern to find all variables inside a value.
+     */
+    public final static Pattern FIND_VARIABLES = Pattern.compile("@\\{([^}]+)}");
+
+    /**
+     * Pattern to extract all variable values from a value.
+     */
+    // FIXME REMOVE
+    public final static Pattern FIND_VALUES = Pattern.compile("(?<=@\\{)([^}]+)(?=})");
+
+}