Integrate plugin update checker library

Added the Plugin Update Checker library under lib/plugin-update-checker and integrated it into WPSlug_Core for automatic update checks. This enables update notifications and management for the plugin via a remote JSON metadata file.

lib/plugin-update-checker/languages/plugin-update-checker-ja.po

@@ -0,0 +1,57 @@
+msgid ""
+msgstr ""
+"Project-Id-Version: \n"
+"POT-Creation-Date: 2019-07-15 17:07+0900\n"
+"PO-Revision-Date: 2019-07-15 17:12+0900\n"
+"Last-Translator: tak <tak7725@gmail.com>\n"
+"Language-Team: \n"
+"Language: ja_JP\n"
+"MIME-Version: 1.0\n"
+"Content-Type: text/plain; charset=UTF-8\n"
+"Content-Transfer-Encoding: 8bit\n"
+"X-Generator: Poedit 2.2.3\n"
+"X-Poedit-Basepath: ../../../../../../Applications/XAMPP/xamppfiles/htdocs/"
+"kisagai/wordpress/wp-content/plugins/simple-stripe-gateway/Puc\n"
+"Plural-Forms: nplurals=1; plural=0;\n"
+"X-Poedit-KeywordsList: __;_x:1,2c\n"
+"X-Poedit-SearchPath-0: .\n"
+
+#: v4p7/Plugin/Ui.php:54
+msgid "View details"
+msgstr "詳細を表示"
+
+#: v4p7/Plugin/Ui.php:77
+#, php-format
+msgid "More information about %s"
+msgstr "%sについての詳細"
+
+#: v4p7/Plugin/Ui.php:128
+msgid "Check for updates"
+msgstr "アップデートを確認"
+
+#: v4p7/Plugin/Ui.php:213
+#, php-format
+msgctxt "the plugin title"
+msgid "The %s plugin is up to date."
+msgstr "%s プラグインは、最新バージョンです。"
+
+#: v4p7/Plugin/Ui.php:215
+#, php-format
+msgctxt "the plugin title"
+msgid "A new version of the %s plugin is available."
+msgstr "%s プラグインの最新バージョンがあります。"
+
+#: v4p7/Plugin/Ui.php:217
+#, php-format
+msgctxt "the plugin title"
+msgid "Could not determine if updates are available for %s."
+msgstr "%s のアップデートがあるかどうかを判断できませんでした。"
+
+#: v4p7/Plugin/Ui.php:223
+#, php-format
+msgid "Unknown update checker status \"%s\""
+msgstr "バージョンアップの確認で想定外の状態になりました。ステータス：”%s”"
+
+#: v4p7/Vcs/PluginUpdateChecker.php:98
+msgid "There is no changelog available."
+msgstr "更新履歴はありません。"