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.
Implement the `WrenBindForeignMethodFn` callback type and `bindForeignMethodFn` configuration field to allow host applications to resolve foreign method declarations at class definition time. Refactor the IO library to use this new binding mechanism instead of the previous `wrenDefineStaticMethod` approach, adding `foreign` keyword support to the lexer and compiler. Introduce `freeCompiler` helper for cleanup, store module names in `ObjModule`, and update the VM to call the binder when processing `CODE_METHOD_INSTANCE` and `CODE_METHOD_STATIC` opcodes for foreign methods.
Separate Wren VM library sources from CLI tool sources by moving them into
src/vm/ and src/cli/ subdirectories respectively. Update the Makefile to use
separate wildcard patterns for each directory, generate distinct object file
paths under build/vm/ and build/cli/, and adjust include paths to src/include.
Also update the Xcode project file to reference the new file locations.
