feat: replace inline C string literals with auto-generated .wren.inc includes

Replace the manually embedded C string constants for builtin Wren modules with
a new build system that automatically generates `.wren.inc` header files from
`.wren` source files. The old `script/generate_builtins.py` is removed and
replaced by `script/wren_to_c_string.py`, which produces standalone include
files with a `PREAMBLE` containing the module name and source path. The Makefile
gains pattern rules to rebuild these `.wren.inc` files whenever the
corresponding `.wren` source changes, and the `builtin` phony target is removed
since the generation is now automatic. The `wren.mk` wildcard lists are updated
to include the new `.wren.inc` files as dependencies. The `timer.c` and
`wren_core.c` source files now `#include` their respective `.wren.inc` headers
instead of defining the string literals inline, and the `timer.wren` trailing
newline is removed.

Makefile

@@ -46,11 +46,6 @@ test: debug
 	@ $(MAKE) -f script/wren.mk MODE=debug test
 	@ ./script/test.py $(suite)
 
-# Take the contents of the scripts under builtin/ and copy them into their
-# respective wren_<name>.c source files.
-builtin:
-	@ ./script/generate_builtins.py
-
 # Generate the Wren site.
 docs:
 	@ ./script/generate_docs.py