Replace ad-hoc string construction using wrenNewString, wrenStringConcat, and sprintf with the new wrenStringFormat helper. This centralizes string formatting logic, reduces code duplication, and eliminates manual length calculations in wren_compiler.c, wren_core.c, wren_value.c, and wren_vm.c. Also introduces CONST_STRING macro for compile-time string literals and changes wrenNewUninitializedString return type from Value to ObjString* for consistency.
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.
