Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/terminal.hh

@@ -0,0 +1,40 @@
+#pragma once
+///@file
+
+#include <limits>
+#include <string>
+
+namespace nix {
+/**
+ * Determine whether ANSI escape sequences are appropriate for the
+ * present output.
+ */
+bool isTTY();
+
+/**
+ * Truncate a string to 'width' printable characters. If 'filterAll'
+ * is true, all ANSI escape sequences are filtered out. Otherwise,
+ * some escape sequences (such as colour setting) are copied but not
+ * included in the character count. Also, tabs are expanded to
+ * spaces.
+ */
+std::string filterANSIEscapes(std::string_view s,
+    bool filterAll = false,
+    unsigned int width = std::numeric_limits<unsigned int>::max());
+
+/**
+ * Recalculate the window size, updating a global variable.
+ *
+ * Used in the `SIGWINCH` signal handler on Unix, for example.
+ */
+void updateWindowSize();
+
+/**
+ * @return the number of rows and columns of the terminal.
+ *
+ * The value is cached so this is quick. The cached result is computed
+ * by `updateWindowSize()`.
+ */
+std::pair<unsigned short, unsigned short> getWindowSize();
+
+}