Add new private native method: Language#field

As the name suggest, it allows us to retrieve the field name by ID.
It currently sees no use in the code, but I have plans to use it in
the constructor to eagerly fetch all the valid field names of a
language.

lib/ch_usi_si_seart_treesitter_Language.cc

@@ -541,6 +541,14 @@ JNIEXPORT jint JNICALL Java_ch_usi_si_seart_treesitter_Language_fields(
   return (jint)ts_language_field_count(language);
 }
 
+JNIEXPORT jstring JNICALL Java_ch_usi_si_seart_treesitter_Language_field(
+  JNIEnv* env, jclass self, jlong languageId, jint fieldId) {
+  TSLanguage* language = (TSLanguage*)languageId;
+  TSFieldId field = (TSFieldId)fieldId;
+  const char* name = ts_language_field_name_for_id(language, field);
+  return env->NewStringUTF(name);
+}
+
 JNIEXPORT jint JNICALL Java_ch_usi_si_seart_treesitter_Language_states(
   JNIEnv* env, jclass self, jlong id) {
   TSLanguage* language = (TSLanguage*)id;

src/main/java/ch/usi/si/seart/treesitter/Language.java

@@ -540,6 +540,7 @@ public static void validate(@NotNull Language language) {
     private static native int symbols(long id);
     private static native Symbol symbol(long languageId, int symbolId);
     private static native int fields(long id);
+    private static native String field(long languageId, int fieldId);
     private static native int states(long id);
 
     long id;