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.

doc/site/core/bool.markdown

@@ -3,6 +3,8 @@
 
 Boolean values. There are two instances, `true` and `false`.
 
+## Methods
+
 ### **!** operator
 
 Returns the logical complement of the value.

doc/site/core/class.markdown

@@ -1,6 +1,8 @@
 ^title Class Class
 ^category core
 
+## Methods
+
 ### **name**
 
 The name of the class.

doc/site/core/fiber.markdown

@@ -13,6 +13,8 @@ fiber is run. Does not immediately start running the fiber.
       IO.print("I won't get printed")
     }
 
+## Static Methods
+
 ### Fiber.**current**
 
 The currently executing fiber.
@@ -69,6 +71,8 @@ Similar to `Fiber.yield` but provides a value to return to the parent fiber's
 
     IO.print(fiber.call()) // "value"
 
+## Methods
+
 ### **call**()
 
 **TODO**

doc/site/core/fn.markdown

@@ -18,6 +18,8 @@ argument](../functions.html#block-arguments) to some other method.
 
 It is a runtime error if `function` is not a function.
 
+## Methods
+
 ### **arity**
 
 The number of arguments the function requires.

doc/site/core/list.markdown

@@ -5,6 +5,8 @@ Extends [Sequence](sequence.html).
 
 An indexable contiguous collection of elements. More details [here](../lists.html).
 
+## Methods
+
 ### **add**(item)
 
 Appends `item` to the end of the list.

doc/site/core/map.markdown

@@ -3,6 +3,8 @@
 
 An associative collection that maps keys to values. More details [here](../maps.html).
 
+## Methods
+
 ### **clear**()
 
 Removes all entries from the map.

doc/site/core/null.markdown

@@ -1,6 +1,8 @@
 ^title Null Class
 ^category core
 
+## Methods
+
 ### **!** operator
 
 Returns `true`, since `null` is considered [false](../control-flow.html#truth).

doc/site/core/num.markdown

@@ -1,6 +1,21 @@
 ^title Num Class
 ^category core
 
+## Static Methods
+
+### Num.**fromString**(value)
+
+Attempts to parse `value` as a decimal literal and return it as an instance of
+`Num`. If the number cannot be parsed `null` will be returned.
+
+It is a runtime error if `value` is not a string.
+
+### Num.**pi**
+
+The value of π.
+
+## Methods
+
 ### **abs**
 
 The absolute value of the number.
@@ -131,14 +146,3 @@ from the beginning number to the ending number not including the ending number.
     IO.print(range.min)         // 1.2
     IO.print(range.max)         // 3.4
     IO.print(range.isInclusive) // false
-
-### Num.**fromString**(value)
-
-Attempts to parse `value` as a decimal literal and return it as an instance of
-`Num`. If the number cannot be parsed `null` will be returned.
-
-It is a runtime error if `value` is not a string.
-
-### Num.**pi**
-
-The value of π.

doc/site/core/object.markdown

@@ -1,6 +1,8 @@
 ^title Object Class
 ^category core
 
+## Methods
+
 ### **!** operator
 
 Returns `false`, since most objects are considered [true](control-flow.html#truth).

doc/site/core/range.markdown

@@ -5,6 +5,8 @@
 
 Extends [Sequence](sequence.html).
 
+## Methods
+
 ### **from**
 
 **TODO**

doc/site/core/sequence.markdown

@@ -6,6 +6,8 @@ core [iterator protocol][] can extend this to get a number of helpful methods.
 
 [iterator protocol]: ../control-flow.html#the-iterator-protocol
 
+## Methods
+
 ### **all**(predicate)
 
 Tests whether all the elements in the sequence pass the `predicate`.

doc/site/core/string.markdown

@@ -31,6 +31,18 @@ on string *return* byte indices too. So, for example, this does what you want:
 In general, methods on strings will work in terms of code units if they can do
 so efficiently, and will otherwise deal in bytes.
 
+## Static Methods
+
+### String.**fromCodePoint**(codePoint)
+
+Creates a new string containing the UTF-8 encoding of `codePoint`.
+
+It is a runtime error if `codePoint` is not an integer between `0` and
+`0x10ffff`, inclusive.
+
+    :::dart
+    String.fromCodePoint(8225) // "‡"
+
 ## Methods
 
 ### **contains**(other)