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/include.mk

@@ -1,4 +1,7 @@
-V0_SRC := v0/main.c v0/util.c v0/token.c v0/parser.c v0/log.c v0/str.c
+include v0/ast/include.mk
+include v0/parser/include.mk
+
+V0_SRC := v0/main.c v0/util.c v0/token.c $(AST_SRC) $(PARSER_SRC) v0/log.c v0/str.c
 
 # V0_TEST must only include `v0/test.c` itself, as all other test C–source files are
 # included directly into `v0/test.c` using `#include "test_xyz.c"`.