feat: add Sequence.each method for side-effect iteration without list creation

Add a new `each` method to the Sequence class that iterates over elements
calling a provided function for each, serving as a side-effect-only
alternative to `map` when the resulting list is not needed. This is
particularly important for the upcoming change where `map` will return a
new sequence instead of mutating in place, as demonstrated by updating
the README and index examples to use `each` with Fiber.yield. Includes
documentation in the Sequence API reference and three new test cases
covering normal iteration, empty sequences, and non-function argument
error handling.

doc/site/core/sequence.markdown

@@ -54,6 +54,13 @@ and counting the number of times the returned value evaluates to `true`.
     [1, 2, 3].count {|n| n > 2} // 1.
     [1, 2, 3].count {|n| n < 4} // 3.
 
+### **each**(function)
+
+Iterates over the sequence, passing each element to the given `function`.
+
+    :::dart
+    ["one", "two", "three"].each {|word| IO.print(word) }
+
 ### **join**(sep)
 
 Returns a string representation of the list. The string representations of the