Freeze strings and constants in pragmas

I'm looking in earnest at Ractor support in Rails. We need some of these
constants that SQLite3 uses to be frozen so that when they're accessed
via Ractors we don't get an error.

This commit just adds frozen string literals, and freezes some constants

Author: tenderlove

lib/sqlite3/pragmas.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "sqlite3/errors"
 
 module SQLite3
@@ -77,26 +79,26 @@ def set_int_pragma(name, value)
     end
 
     # The enumeration of valid synchronous modes.
-    SYNCHRONOUS_MODES = [["full", 2], ["normal", 1], ["off", 0]]
+    SYNCHRONOUS_MODES = [["full", 2], ["normal", 1], ["off", 0]].map(&:freeze).freeze
 
     # The enumeration of valid temp store modes.
-    TEMP_STORE_MODES = [["default", 0], ["file", 1], ["memory", 2]]
+    TEMP_STORE_MODES = [["default", 0], ["file", 1], ["memory", 2]].map(&:freeze).freeze
 
     # The enumeration of valid auto vacuum modes.
-    AUTO_VACUUM_MODES = [["none", 0], ["full", 1], ["incremental", 2]]
+    AUTO_VACUUM_MODES = [["none", 0], ["full", 1], ["incremental", 2]].map(&:freeze).freeze
 
     # The list of valid journaling modes.
     JOURNAL_MODES = [["delete"], ["truncate"], ["persist"], ["memory"],
-      ["wal"], ["off"]]
+      ["wal"], ["off"]].map(&:freeze).freeze
 
     # The list of valid locking modes.
-    LOCKING_MODES = [["normal"], ["exclusive"]]
+    LOCKING_MODES = [["normal"], ["exclusive"]].map(&:freeze).freeze
 
     # The list of valid encodings.
-    ENCODINGS = [["utf-8"], ["utf-16"], ["utf-16le"], ["utf-16be"]]
+    ENCODINGS = [["utf-8"], ["utf-16"], ["utf-16le"], ["utf-16be"]].map(&:freeze).freeze
 
     # The list of valid WAL checkpoints.
-    WAL_CHECKPOINTS = [["passive"], ["full"], ["restart"], ["truncate"]]
+    WAL_CHECKPOINTS = [["passive"], ["full"], ["restart"], ["truncate"]].map(&:freeze).freeze
 
     def application_id
       get_int_pragma "application_id"