Implemented scopes

tests/integrations/test_variables.toy

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+//declare a variable with an initial value
+var answer = 42;
+
+//declare a variable without an initial value
+var empty;
+
+//assign a previously existing variable
+answer = 6 * 9;
+
+//access a variable
+answer = answer + 1;
+
+//compound assignments
+answer += 5;
+answer -= 5;
+answer *= 9;
+answer /= 2;
+answer %= 10;
+