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.
Refactor wrenFindVariable to accept an explicit ObjModule* parameter instead of
implicitly using the main module. Promote the internal getCoreModule helper to
a public wrenGetCoreModule function declared in wren_vm.h. Update all call sites
in wren_core.c and wren_meta.c to pass the core module when looking up core
variables, and replace inline getCoreModule calls in wren_vm.c with the new
public function. Remove the NULL-module fallback documentation from
wrenDeclareVariable and wrenDefineVariable.
Add foreign static eval(source) method to Meta class in meta.wren and implement wrenBindMetaForeignMethod in wren_meta.c to handle its binding. Change the implicit module name from "main" to "core" in wren_vm.c and restrict foreign method lookup to only the "core" module, enabling Meta foreign methods to be resolved alongside IO methods. Relax the IO foreign method binder to return NULL instead of asserting for non-IO classes, allowing fallback to other library binders.
Add wren_meta.c and wren_meta.h file references to the Xcode project.pbxproj, including the wren_meta.c source in the PBXBuildFile section and both files in the PBXFileReference section. Also reformat indentation in metaEval function body and add a TODO comment for argument type checking.
