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/declaration.h

@@ -0,0 +1,49 @@
+#ifndef AST_DECLARATION_H
+#define AST_DECLARATION_H
+
+#include "expression.h"
+#include "../bool.h"
+
+typedef struct {
+    /** @brief The name of the module being imported. */
+    char* module_name;
+
+    /** @brief Whether the import is public or not. */
+    bool is_public;
+} ImportDeclaration;
+
+/**
+ * A declaration that aliases one type to another.
+ */
+typedef struct {
+    /** @brief The name of the alias. */
+    const char* name;
+
+    /** @brief The value of the alias. */
+    TypeExpression value;
+} AliasDeclaration;
+
+/**
+ * A declaration of a variable, which may be a constant or not, and may be static or not.
+ */
+typedef struct {
+    /** @brief The name of the variable. */
+    char* name;
+
+    /** @brief The type of the variable. */
+    TypeExpression type;
+
+    /** @brief The optional initializer expression. */
+    Expression* initializer;
+
+    /** @brief Whether the variable is public or not. */
+    bool is_public;
+
+    /** @brief Whether the variable is static or not. */
+    bool is_static;
+
+    /** @brief Whether the variable is a constant or not. */
+    bool is_const;
+} VariableDeclaration;
+
+#endif