jinja: unary minus before a variable fails to parse #51

Upstream: ggml-org#29233

Dynamic negative slice bounds such as items[:-n] fail to parse, while negative integer literals work.

Reproduced on spark-a934.local, Linux aarch64, upstream 58367713a6935c0810103378144008df32e3d5db, CPU Release llama-common. No model required.

#include "jinja/lexer.h"
#include "jinja/parser.h"
#include <iostream>
int main() {
    for (const char * s : {"{{items[:-n]|string}}", "{{items[:-1]|string}}"}) {
        try {
            jinja::lexer lexer;
            auto tokens = lexer.tokenize(s);
            auto program = jinja::parse_from_tokens(tokens);
            std::cout << "parsed\n";
        } catch (const std::exception & e) {
            std::cout << e.what() << '\n';
        }
    }
}

From a checkout with llama-common built in build/bin:

g++ -std=c++17 -Icommon repro.cpp -Lbuild/bin -Wl,-rpath,"$PWD/build/bin" -lllama-common -o repro
./repro

Observed: Unexpected token: - of type 24 for the variable-bound template; parsed for the literal control. Failure is at parse time, before variable evaluation. Introducing commit not bisected.