Refactor AST and Parser into modular subdirectories

- Split ast.h into granular headers in v0/ast/
- Split parser.c into modular implementation files in v0/parser/
- Move and rename parser tests to v0/parser/test_*.c
- Update build system (include.mk) with modular sub-makefiles
- Maintain v0/ast.h and v0/parser.h as umbrella headers

v0/ast/include.mk

@@ -0,0 +1,3 @@
+# There are currently no .c files in the ast directory.
+# This file is provided for future consistency.
+AST_SRC :=