add recording_register value expansion

Author: evnoj

doc/pages/expansions.asciidoc

@@ -334,6 +334,9 @@ The following expansions are supported (with required context _in italics_):
     an inner selection, `to_begin` if the user wants to select to the
     beginning, and `to_end` if the user wants to select to the end
 
+*%val{recording_register}*::
+    the register being recorded to if currently recording a macro, empty when not recording
+
 *%val{register}*::
     _in `map` command <keys> parameter and `<a-;>` from the object menu_ +
     current register when the mapping was triggered

src/main.cc

@@ -257,6 +257,11 @@ const EnvVarDesc builtin_env_vars[] = { {
         [](StringView name, const Context& context) -> Vector<String>
         { auto cursor = context.selections().main().cursor();
           return {to_string(context.buffer().distance({0,0}, cursor))}; }
+    }, {
+        "recording_register", false,
+        [](StringView name, const Context& context) -> Vector<String>
+        { const char reg = context.client().input_handler().recording_reg();
+          return {reg ? String{static_cast<Codepoint>(reg)} : ""}; }
     }, {
         "selection_desc", false,
         [](StringView name, const Context& context) -> Vector<String>