feat: add Sequence.count(predicate) method and update documentation

Implement a new `count(f)` method on the Sequence class that accepts a predicate function and returns the number of elements for which the predicate evaluates to true. The implementation iterates over the sequence, calling `f.call(element)` on each element and incrementing a counter when the result is truthy.

Also update the documentation for both Sequence and List classes: add full documentation for the new `count(predicate)` method on Sequence, and change the terminology in List docs from "items" to "elements" for consistency. Add three test files covering the basic predicate behavior, non-boolean return values from the predicate, and the error case when a non-function argument is passed.

test/core/list/count_predicate.wren

@@ -0,0 +1,6 @@
+var a = [1, 2, 3]
+
+IO.print(a.count {|x| x > 3 })      // expect: 0
+IO.print(a.count {|x| x > 1 })      // expect: 2
+
+IO.print([].count {|x| true })      // expect: 0

test/core/list/count_predicate_non_bool_returning_fn.wren

@@ -0,0 +1,3 @@
+var a = [1, 2, 3]
+
+IO.print(a.count {|x| "truthy" })   // expect: 3

test/core/list/count_predicate_non_function_arg.wren

@@ -0,0 +1,3 @@
+var a = [1, 2, 3]
+
+a.count("string") // expect runtime error: String does not implement 'call(_)'.