llama : add remove_space_prefix to llama_detokenize

This commit adds a new parameter to llama_detokenize to remove the
leading space before tokens if they have a word boundary character.

The motivation for this change is that when llama_server returns
completion_propabilities, the tokens are detokenized and currently
the leading space for the boundary tokens are removed. With this change
llama_server can set remove_space_prefix to false and the leading space
will be preserved.

Resolves: https://github.com/ggerganov/llama.cpp/issues/11728

examples/server/server.cpp

@@ -2297,7 +2297,7 @@ struct server_context {
             for (size_t i = 0; i < std::min(n_vocab, n_probs); i++) {
                 result.probs.push_back({
                     cur[i].id,
-                    common_detokenize(ctx, {cur[i].id}, special),
+                    common_detokenize(ctx, {cur[i].id}, special, /* remove_space_prefix */ false),
                     cur[i].p
                 });
             }