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.

builtin/README.md

@@ -1,7 +1,9 @@
-The Wren scripts in this directory get converted to C string literals and then
-inserted into their respective .c files so that the interpreter can load them
-directly without having to do any file IO.
+The Wren scripts in this directory get converted to C string literals into files
+with a `.wren.inc` extension. Those are then `#includ`ed into their respective
+`.c` files so that the interpreter can load them directly without having to do
+any file IO.
 
-The script that does this copying is `script/generate_builtins.py`.
+The script that does this translation is `script/wren_to_c_string.py`.
 
-You can invoke using `make builtin`.
+When any of the ".wren" files in here are changed, the Makefile automatically
+updates the generated C headers.