feat: add split and replace methods to String class with tests and docs

Implement String.split(delim) and String.replace(from, to) methods in the core Wren VM. split returns a list of substrings separated by the given delimiter, while replace returns a new string with all occurrences of the old substring replaced by the new one. Both methods validate arguments (must be non-empty strings) and abort the fiber on invalid input. Include comprehensive test suites covering basic usage, multiple matches, non-ASCII characters, 8-bit clean data, and error cases. Update the string.markdown documentation with usage examples for both methods.

doc/site/modules/core/string.markdown

@@ -129,6 +129,24 @@ negative to count backwards from the end of the string.
 It is a runtime error if `search` is not a string or `start` is not an integer
 index within the string's byte length.
 
+### **split**(seperator)
+
+Returns a list of one or more strings seperated by `seperator`.
+
+    :::wren
+    var string = "abc abc abc"
+    System.print(string.split(" ")) //> [abc, abc, abc]
+
+It is a runtime error if `seperator` is not a string or is an empty string.
+
+### **replace**(old, swap)
+
+Returns a new string with all occurences of `old` replaced with `swap`.
+
+    :::wren
+    var string = "abc abc abc"
+    System.print(string.replace(" ", "")) //> abcabcabc
+
 ### **iterate**(iterator), **iteratorValue**(iterator)
 
 Implements the [iterator protocol](../../control-flow.html#the-iterator-protocol)