Add error when a terminal symbol is passed to %nterm

In bison when a terminal symbol is passed to `%nterm` it should give an error. But it doesn't yet.
It just ignores the terminal symbol and uses the nonterminal symbol.

For example in the following code, `EOI` is a terminal symbol and `EOI` is a nonterminal symbol. When `EOI` is passed to `%nterm` it should give an error but it doesn't.

```yacc
%{
// Prologue
%}

%token EOI 0 "EOI"
%nterm EOI

%%

program: /* empty */
        ;
```

In bison, it gives the following error

```
❯ bison test.y
test.y:6.8-10: error: symbol EOI redeclared as a nonterminal
    6 | %nterm EOI
      |        ^~~
test.y:5.8-10: note: previous definition
    5 | %token EOI 0 "EOI"
      |        ^~~
```

So this PR adds an error when a terminal symbol is passed to `%nterm`.

lib/lrama/grammar.rb

@@ -30,7 +30,7 @@ class Grammar
                   :after_shift, :before_reduce, :after_reduce, :after_shift_error_token, :after_pop_stack,
                   :symbols_resolver, :types, :rules, :rule_builders, :sym_to_rules, :no_stdlib, :locations
 
-    def_delegators "@symbols_resolver", :symbols, :nterms, :terms, :add_nterm, :add_term,
+    def_delegators "@symbols_resolver", :symbols, :nterms, :terms, :add_nterm, :add_term, :find_term_by_s_value,
                                         :find_symbol_by_number!, :find_symbol_by_id!, :token_to_symbol,
                                         :find_symbol_by_s_value!, :fill_symbol_number, :fill_nterm_type,
                                         :fill_printer, :fill_destructor, :fill_error_token, :sort_by_number!

lib/lrama/grammar/symbols/resolver.rb

@@ -57,6 +57,10 @@ def add_nterm(id:, alias_name: nil, tag: nil)
           nterm
         end
 
+        def find_term_by_s_value(s_value)
+          terms.find { |s| s.id.s_value == s_value }
+        end
+
         def find_symbol_by_s_value(s_value)
           symbols.find { |s| s.id.s_value == s_value }
         end