Remove the deferred function compilation for interpolation parts and instead evaluate them directly during string parsing. This simplifies the compiler by eliminating the need for String.interpolate_() and the associated closure creation, replacing it with a direct list construction and join() call.
Add lexer support for `TOKEN_INTERPOLATION` to split string literals at interpolation points, introduce `MAX_INTERPOLATION_NESTING` limit of 8, and compile interpolated strings by emitting calls to `String.interpolate_()`. Optimize list and map literal construction with new `addCore_` primitives to reduce stack churn. Update `String`, `List`, and `Map` `toString` methods in core library to use interpolation syntax, and migrate all benchmark and test files from explicit concatenation to interpolated strings.
- Remove builtin/README.md explaining the .wren to .wren.inc conversion process
- Update src/README.md to document new aux/ directory for optional modules
- Fix generated file headers in wren_core.wren.inc and wren_aux_random.wren.inc to point to new source locations
- Update Makefile pattern rule to look for .wren sources in src/vm/ instead of builtin/
- Adjust wren_to_c_string.py to strip both "aux_" and "wren_" prefixes when deriving module names
