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.

util/pygments-lexer/setup.py

@@ -0,0 +1,18 @@
+"""
+A Pygments lexer for Wren.
+"""
+from setuptools import setup
+
+__author__ = 'Robert Nystrom'
+
+setup(
+    name='Wren',
+    version='1.0',
+    description=__doc__,
+    author=__author__,
+    packages=['wren'],
+    entry_points='''
+    [pygments.lexers]
+    wrenlexer = wren:WrenLexer
+    '''
+)