Add the missing `tan` method to the Num class, implementing the tangent
trigonometric function. This fills the gap between the existing `sin` and
`cos` methods, providing a complete set of basic trigonometric operations.
- Register `num_tan` primitive in the VM core initialization
- Implement `DEF_PRIMITIVE(num_tan)` using C's `tan()` math function
- Add documentation entry for `tan` in the core library reference
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.
