Add text mode handling.

src/internal/functions/functions.cc

@@ -53,6 +53,24 @@ void Function::register_posix(BEd &ctx) {
       throw fatal_error("Quitting", 0);
     }
   };
+  ctx.functions.insert(
+    "a",
+    Function{
+      .address_kind = Function::AddressKind::Line,
+      .argument_kind = Function::ArgumentKind::None,
+      .input_mode = Function::InputMode::Text,
+      .desc = "Append text to a line.",
+      .default_address = ".",
+      .accept_zero = true,
+      .pre_text_mode = nullptr,
+      .handle = [](BEd &ctx, const buffer::Address &addr_, vase::Shard *text, const Argument &, std::vector<buffer::Line> *) {
+        auto addr = std::get<buffer::Line>(addr_);
+        ctx.buffer(addr.buffername).append(ctx, text, addr.number);
+        ctx.current() = {ctx.prev.buffername, ctx.prev.end};
+        vase::Shard::release(text);
+      }
+    }
+  );
   ctx.functions.insert(
     "j",
     Function{