tools/build_emoji: Generate `name_to_codepoint.json` file.

Generate the `name_to_codepoint.json` file in build_emoji which will be
used by bugdown/ to map emoji names to codepoints directly.

tools/setup/emoji/build_emoji

@@ -252,5 +252,11 @@ def dump_emojis(cache_path):
     })
     emoji_codes_file.close()
 
+    NAME_TO_CODEPOINT_PATH = os.path.join(cache_path, 'name_to_codepoint.json')
+    name_to_codepoint_file = open(NAME_TO_CODEPOINT_PATH, 'w')
+
+    name_to_codepoint_file.write(json.dumps(emoji_map))
+    name_to_codepoint_file.close()
+
 if __name__ == "__main__":
     main()