Add initial unit tests

- Add µnit source and header files
- Add test target to the build system
- Implement a thorough lexer test suite
- Implement a minimal AST test suite

tests/main.c

@@ -0,0 +1,16 @@
+#include "munit.h"
+
+extern MunitTest ast_tests[];
+extern MunitTest lexer_tests[];
+
+int main(int argc, char *argv[MUNIT_ARRAY_PARAM(argc + 1)]) {
+    MunitSuite suites[] = {
+        {"/ast",   ast_tests,   nullptr, 1, MUNIT_SUITE_OPTION_NONE},
+        {"/lexer", lexer_tests, nullptr, 1, MUNIT_SUITE_OPTION_NONE},
+        {nullptr,  nullptr,     nullptr, 0, MUNIT_SUITE_OPTION_NONE},
+    };
+
+    MunitSuite master_suite = {"/oas", nullptr, suites, 1, MUNIT_SUITE_OPTION_NONE};
+
+    return munit_suite_main(&master_suite, nullptr, argc, argv);
+}