Move hiliteMatches into a separate header

This is mostly so that we don't #include <regex> everywhere (which
adds quite a bit of compilation time).

src/libutil/hilite.hh

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <regex>
+#include <vector>
+#include <string>
+
+namespace nix {
+
+/* Highlight all the given matches in the given string `s` by wrapping
+   them between `prefix` and `postfix`.
+
+   If some matches overlap, then their union will be wrapped rather
+   than the individual matches. */
+std::string hiliteMatches(
+    std::string_view s,
+    std::vector<std::smatch> matches,
+    std::string_view prefix,
+    std::string_view postfix);
+
+}