chore: move core.wren source from builtin/ to src/vm/ and update build paths

- Remove builtin/README.md explaining the .wren to .wren.inc conversion process
- Update src/README.md to document new aux/ directory for optional modules
- Fix generated file headers in wren_core.wren.inc and wren_aux_random.wren.inc to point to new source locations
- Update Makefile pattern rule to look for .wren sources in src/vm/ instead of builtin/
- Adjust wren_to_c_string.py to strip both "aux_" and "wren_" prefixes when deriving module names

util/wren.mk

@@ -208,7 +208,7 @@ $(LIBUV): $(LIBUV_DIR)/build/gyp/gyp util/libuv.py
 src/aux/wren_aux_%.wren.inc: src/aux/wren_aux_%.wren util/wren_to_c_string.py
 	@ ./util/wren_to_c_string.py $@ $<
 
-src/vm/wren_%.wren.inc: builtin/%.wren util/wren_to_c_string.py
+src/vm/wren_%.wren.inc: src/vm/wren_%.wren util/wren_to_c_string.py
 	@ ./util/wren_to_c_string.py $@ $<
 
 src/module/%.wren.inc: src/module/%.wren util/wren_to_c_string.py

util/wren_to_c_string.py

@@ -42,7 +42,8 @@ def main():
     wren_source_lines = f.readlines()
 
   module = os.path.splitext(os.path.basename(args.input))[0]
-  module = module.replace("wren_aux_", "")
+  module = module.replace("aux_", "")
+  module = module.replace("wren_", "")
 
   c_source = wren_to_c_string(args.input, wren_source_lines, module)