Extract the static `readFile`, `readModule`, and `setRootDirectory` functions from main.c into a new io module, replacing the fixed-size `rootDirectory` array with a heap-allocated pointer and adding the corresponding header with public declarations.
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.
