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/tests/parser_alias_simple_type.c2

@@ -0,0 +1,3 @@
+module mymodule;
+
+alias myalias = i32;

v0/tests/parser_variable_simple_type.c2

@@ -0,0 +1,4 @@
+module my_module;
+
+// Defines a global variable called my_var.
+i32 my_var;