Add source code.

include/Completion.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include <cstddef>
+#include <string>
+#include <vector>
+
+// Result of a tab-completion request: the text from `start` up to the cursor
+// can be replaced by any of `candidates`.
+struct Completion {
+    std::size_t start = 0;
+    std::vector<std::string> candidates;
+};