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/syntax.markdown

@@ -13,13 +13,13 @@ bytecode for efficiency, but that's an implementation detail.)
 
 Line comments start with `//` and end at the end of the line:
 
-    :::dart
+    :::wren
     // This is a comment.
 
 Block comments start with `/*` and end with `*/`. They can span multiple lines
 or be within a single one. Unlike C, block comments can nest in Wren:
 
-    :::dart
+    :::wren
     /* This is /* a nested */ comment. */
 
 ## Reserved words
@@ -27,7 +27,7 @@ or be within a single one. Unlike C, block comments can nest in Wren:
 Some people like to see all of the reserved words in a programming language in
 one lump. If you're one of those folks, here you go:
 
-    :::dart
+    :::wren
     break class construct else false for foreign if import
     in is null return static super this true var while
 
@@ -37,7 +37,7 @@ Naming rules are similar to other programming languages. Identifiers start with
 a letter or underscore and may contain letters, digits, and underscores. Case
 is sensitive.
 
-    :::dart
+    :::wren
     hi
     camelCase
     PascalCase
@@ -52,7 +52,7 @@ to indicate [fields](classes.html#fields) in classes.
 
 Newlines (`\n`) are meaningful in Wren. They are used to separate statements:
 
-    :::dart
+    :::wren
     // Two statements:
     System.print("hi") // Newline.
     System.print("bye")
@@ -61,7 +61,7 @@ Sometimes, though, a statement doesn't fit on a single line and jamming a
 newline in the middle would trip it up. To handle that, Wren has a very simple
 rule: It ignores a newline following any token that can't end a statement.
 
-    :::dart
+    :::wren
     System.print( // Newline here is ignored.
         "hi")
 
@@ -76,7 +76,7 @@ statement is allowed, like in [control flow](control-flow.html) statements.
 blocks. For example, here we have a block for the then case, and a single
 statement for the else:
 
-    :::dart
+    :::wren
     if (happy && knowIt) {
       hands.clap
     } else System.print("sad")
@@ -84,7 +84,7 @@ statement for the else:
 Blocks have two similar but not identical forms. Typically, blocks contain a
 series of statements like:
 
-    :::dart
+    :::wren
     {
       System.print("one")
       System.print("two")
@@ -99,7 +99,7 @@ However, it's pretty common to have a method or function that just evaluates
 and returns the result of a single expression. For that, Wren has a more
 compact notation:
 
-    :::dart
+    :::wren
     { "single expression" }
 
 If there is no newline after the `{` (or after the parameter list in a
@@ -107,7 +107,7 @@ If there is no newline after the `{` (or after the parameter list in a
 expression, and it automatically returns the result of it. It's exactly the
 same as doing:
 
-    :::dart
+    :::wren
     {
       return "single expression"
     }