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/Puc/v5p3/InstalledPackage.php

@@ -0,0 +1,105 @@
+<?php
+namespace YahnisElsts\PluginUpdateChecker\v5p3;
+
+if ( !class_exists(InstalledPackage::class, false) ):
+
+	/**
+	 * This class represents a currently installed plugin or theme.
+	 *
+	 * Not to be confused with the "package" field in WP update API responses that contains
+	 * the download URL of a the new version.
+	 */
+	abstract class InstalledPackage {
+		/**
+		 * @var UpdateChecker
+		 */
+		protected $updateChecker;
+
+		public function __construct($updateChecker) {
+			$this->updateChecker = $updateChecker;
+		}
+
+		/**
+		 * Get the currently installed version of the plugin or theme.
+		 *
+		 * @return string|null Version number.
+		 */
+		abstract public function getInstalledVersion();
+
+		/**
+		 * Get the full path of the plugin or theme directory (without a trailing slash).
+		 *
+		 * @return string
+		 */
+		abstract public function getAbsoluteDirectoryPath();
+
+		/**
+		 * Check whether a regular file exists in the package's directory.
+		 *
+		 * @param string $relativeFileName File name relative to the package directory.
+		 * @return bool
+		 */
+		public function fileExists($relativeFileName) {
+			return is_file(
+				$this->getAbsoluteDirectoryPath()
+				. DIRECTORY_SEPARATOR
+				. ltrim($relativeFileName, '/\\')
+			);
+		}
+
+		/* -------------------------------------------------------------------
+		 * File header parsing
+		 * -------------------------------------------------------------------
+		 */
+
+		/**
+		 * Parse plugin or theme metadata from the header comment.
+		 *
+		 * This is basically a simplified version of the get_file_data() function from /wp-includes/functions.php.
+		 * It's intended as a utility for subclasses that detect updates by parsing files in a VCS.
+		 *
+		 * @param string|null $content File contents.
+		 * @return string[]
+		 */
+		public function getFileHeader($content) {
+			$content = (string)$content;
+
+			//WordPress only looks at the first 8 KiB of the file, so we do the same.
+			$content = substr($content, 0, 8192);
+			//Normalize line endings.
+			$content = str_replace("\r", "\n", $content);
+
+			$headers = $this->getHeaderNames();
+			$results = array();
+			foreach ($headers as $field => $name) {
+				$success = preg_match('/^[ \t\/*#@]*' . preg_quote($name, '/') . ':(.*)$/mi', $content, $matches);
+
+				if ( ($success === 1) && $matches[1] ) {
+					$value = $matches[1];
+					if ( function_exists('_cleanup_header_comment') ) {
+						$value = _cleanup_header_comment($value);
+					}
+					$results[$field] = $value;
+				} else {
+					$results[$field] = '';
+				}
+			}
+
+			return $results;
+		}
+
+		/**
+		 * @return array Format: ['HeaderKey' => 'Header Name']
+		 */
+		abstract protected function getHeaderNames();
+
+		/**
+		 * Get the value of a specific plugin or theme header.
+		 *
+		 * @param string $headerName
+		 * @return string Either the value of the header, or an empty string if the header doesn't exist.
+		 */
+		abstract public function getHeaderValue($headerName);
+
+	}
+endif;