feat: replace all :::dart code fence markers with :::wren in documentation

Add a custom Pygments lexer plug-in for Wren syntax highlighting, update contributing docs with setup instructions for the lexer, and switch every `:::dart` annotation across all doc/site markdown files to `:::wren` so code examples render with the correct language grammar.

doc/site/core/num.markdown

@@ -20,7 +20,7 @@ The value of π.
 
 The absolute value of the number.
 
-    :::dart
+    :::wren
     -123.abs // 123
 
 ### **acos**
@@ -44,7 +44,7 @@ numbers to determine the quadrant of the result.
 
 Rounds the number up to the nearest integer.
 
-    :::dart
+    :::wren
     1.5.ceil // 2
     (-3.2).ceil // -3
 
@@ -56,7 +56,7 @@ The cosine of the number.
 
 Rounds the number down to the nearest integer.
 
-    :::dart
+    :::wren
     1.5.floor    // 1
     (-3.2).floor // -4
 
@@ -82,7 +82,7 @@ The tangent of the number.
 
 Negates the number.
 
-    :::dart
+    :::wren
     var a = 123
     -a // -123
 
@@ -130,7 +130,7 @@ It is a runtime error if `other` is not a number.
 Creates a [Range](core/range.html) representing a consecutive range of numbers
 from the beginning number to the ending number.
 
-    :::dart
+    :::wren
     var range = 1.2..3.4
     System.print(range.min)         // 1.2
     System.print(range.max)         // 3.4
@@ -141,7 +141,7 @@ from the beginning number to the ending number.
 Creates a [Range](core/range.html) representing a consecutive range of numbers
 from the beginning number to the ending number not including the ending number.
 
-    :::dart
+    :::wren
     var range = 1.2...3.4
     System.print(range.min)         // 1.2
     System.print(range.max)         // 3.4