Merge remote-tracking branch 'occivink/add-selection-count-value'

doc/pages/expansions.asciidoc

@@ -373,6 +373,10 @@ The following expansions are supported (with required context _in italics_):
     _in window scope_ +
     unquoted list of the lengths (in codepoints) of the selections
 
+*%val{selection_count}*::
+    _in window scope_ +
+    the number of selections
+
 *%val{session}*::
     name of the current session
 

src/main.cc

@@ -349,6 +349,10 @@ static const EnvVarDesc builtin_env_vars[] = { {
                      transform([&](const Selection& s) -> String {
                          return to_string(char_length(context.buffer(), s));
                      }) | gather<Vector<String>>(); }
+    }, {
+        "selection_count", false,
+        [](StringView name, const Context& context) -> Vector<String>
+        { return {to_string(context.selections().size())}; }
     }, {
         "window_width", false,
         [](StringView name, const Context& context) -> Vector<String>