Enable grammar parsing

[jwijffels]

Currently not integrated in whisper_encode

Removed from main.cpp the following snippet

if (!params.grammar.empty()) {
        auto & grammar = params.grammar_parsed;
        if (is_file_exist(params.grammar.c_str())) {
            // read grammar from file
            std::ifstream ifs(params.grammar.c_str());
            const std::string txt = std::string((std::istreambuf_iterator<char>(ifs)), std::istreambuf_iterator<char>());
            grammar = grammar_parser::parse(txt.c_str());
        } else {
            // read grammar from string
            grammar = grammar_parser::parse(params.grammar.c_str());
        }

        // will be empty (default) if there are parse errors
        if (grammar.rules.empty()) {
            fprintf(stderr, "error: failed to parse grammar \"%s\"\n", params.grammar.c_str());
            return 4;
        } else {
            fprintf(stderr, "%s: grammar:\n", __func__);
            grammar_parser::print_grammar(stderr, grammar);
            fprintf(stderr, "\n");
        }
    }

as well as the following grammar-related relevant snippet

const auto & grammar_parsed = params.grammar_parsed;
            auto grammar_rules = grammar_parsed.c_rules();

            if (use_grammar) {
                if (grammar_parsed.symbol_ids.find(params.grammar_rule) == grammar_parsed.symbol_ids.end()) {
                    fprintf(stderr, "%s: warning: grammar rule '%s' not found - skipping grammar sampling\n", __func__, params.grammar_rule.c_str());
                } else {
                    wparams.grammar_rules = grammar_rules.data();
                    wparams.n_grammar_rules = grammar_rules.size();
                    wparams.i_start_rule = grammar_parsed.symbol_ids.at(params.grammar_rule);
                    wparams.grammar_penalty = params.grammar_penalty;
                }
            }