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.

test/core/string/replace.wren

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+System.print("something".replace("some", "no")) // expect: nothing
+System.print("something".replace("thing", "one")) // expect: someone
+System.print("something".replace("ometh", "umm"))  // expect: summing
+System.print("something".replace("math", "ton")) // expect: something
+
+// Multiple.
+System.print("somethingsomething".replace("some", "no")) // expect: nothingnothing
+System.print("abc abc abc".replace(" ", "")) // expect: abcabcabc
+System.print("abcabcabc".replace("abc", "")) // expect: 
+
+// Non-ASCII.
+System.print("søméthîng".replace("sømé", "nø"))  // expect: nøthîng
+System.print("søméthîng".replace("meth", "ton")) // expect: søméthîng
+
+// 8-bit clean.
+System.print("a\0b\0c".replace("\0", "")) // expect: abc
+System.print("a\0b\0c".replace("b", "") == "a\0\0c") // expect: true

test/core/string/replace_empty_old.wren

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+"foo".replace("", "f") // expect runtime error: From must be a non-empty string.

test/core/string/replace_new_not_string.wren

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+"foo".replace("o", 1) // expect runtime error: To must be a string.

test/core/string/replace_old_not_string.wren

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+"foo".replace(1, "o") // expect runtime error: From must be a non-empty string.

test/core/string/split.wren

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+System.print("something".split("meth")) // expect: [so, ing]
+System.print("something".split("some")) // expect: [, thing]
+System.print("something".split("ing"))  // expect: [someth, ]
+System.print("something".split("math")) // expect: [something]
+
+// Multiple.
+System.print("somethingsomething".split("meth")) // expect: [so, ingso, ing]
+System.print("abc abc abc".split(" ")) // expect: [abc, abc, abc]
+System.print("abcabcabc".split("abc")) // expect: [, , , ]
+
+// Non-ASCII.
+System.print("søméthîng".split("méth"))  // expect: [sø, îng]
+System.print("søméthîng".split("meth")) // expect: [søméthîng]
+
+// 8-bit clean.
+System.print("a\0b\0c".split("\0")) // expect: [a, b, c]

test/core/string/split_argument_not_string.wren

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+"foo".split(1) // expect runtime error: Argument must be a non-empty string.

test/core/string/split_empty_seperator.wren

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+"foo".split("") // expect runtime error: Argument must be a non-empty string.