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/expression.h

@@ -0,0 +1,52 @@
+#ifndef AST_EXPRESSION_H
+#define AST_EXPRESSION_H
+
+#include "../bool.h"
+
+typedef enum {
+    EXPRESSION_INTEGER,
+    EXPRESSION_STRING,
+    EXPRESSION_BOOLEAN
+} ExpressionTag;
+
+typedef struct {
+    ExpressionTag tag;
+    union {
+        int integer;
+        const char* string;
+        bool boolean;
+    };
+} Expression;
+
+typedef enum {
+    TYPE_EXPRESSION_BUILTIN,
+    TYPE_EXPRESSION_ARRAY
+} TypeExpressionTag;
+
+/**
+ * An expression that evaluates to a type.
+ */
+typedef struct TypeExpression TypeExpression;
+struct TypeExpression {
+    /** @brief defines which entry in the union is valid */
+    TypeExpressionTag tag;
+    union {
+        /** @brief Evaluates to an array of the given type. */
+        struct {
+            /** @brief A pointer to the type of the elements stored in the array. */
+            TypeExpression* array;
+        } array;
+        /** @brief Evaluates to a builtin integer type.*/
+        struct {
+            /**
+             * @brief The number of bits in the integer.
+             * Typical values are 8, 16, 32, and 64.
+             */
+            int bitSize;
+            /** @brief `true` if the type is signed, `false` if it's unsigned. */
+            bool isSigned;
+        } builtin;
+    };
+};
+
+#endif