feat: add String.fromCodePoint static method and refactor UTF-8 encoding into reusable utilities

Implement the `String.fromCodePoint(_)` primitive that creates a string from a Unicode code point, with validation for integer range 0–0x10ffff. Extract inline UTF-8 encoding logic from `readUnicodeEscape` in the compiler into new `wrenUtf8NumBytes` and `wrenUtf8Encode` utility functions in `wren_utils`, and add `wrenStringFromCodePoint` helper in `wren_value`. Update documentation for core classes to add "Methods" and "Static Methods" section headers, and include `String.fromCodePoint` docs with example. Add a test file `from_code_point.wren` covering various code points.

test/core/string/from_code_point.wren

@@ -0,0 +1,5 @@
+IO.print(String.fromCodePoint(65))   // expect: A
+IO.print(String.fromCodePoint(164))   // expect: ¤
+IO.print(String.fromCodePoint(398))   // expect: Ǝ
+IO.print(String.fromCodePoint(8225))   // expect: ‡
+IO.print(String.fromCodePoint(0x254b))   // expect: ╋

test/core/string/from_code_point_not_int.wren

@@ -0,0 +1 @@
+IO.print(String.fromCodePoint(12.34))   // expect runtime error: Code point must be an integer.

test/core/string/from_code_point_not_num.wren

@@ -0,0 +1 @@
+IO.print(String.fromCodePoint("not num"))   // expect runtime error: Code point must be a number.

test/core/string/from_code_point_too_large.wren

@@ -0,0 +1,3 @@
+// UTF-8 mandates that only values up to 10ffff can be encoded.
+// See: http://tools.ietf.org/html/rfc3629
+IO.print(String.fromCodePoint(0x10ffff + 1))   // expect runtime error: Code point cannot be greater than 0x10ffff.

test/core/string/from_code_point_too_small.wren

@@ -0,0 +1 @@
+IO.print(String.fromCodePoint(-1))   // expect runtime error: Code point cannot be negative.