Add documentation comments for PurlError and PackageURL classes

Adds JSDoc comments to improve code documentation:
- PurlError class: Documents custom error class for PURL failures
- PackageURL class: Documents main PURL parser and constructor

These additions ensure all major classes have descriptive comments
explaining their purpose and functionality.

Author: jdalton

src/error.ts

@@ -28,6 +28,9 @@ function formatPurlErrorMessage(message = ''): string {
   return `Invalid purl: ${formatted}`
 }
 
+/**
+ * Custom error class for Package URL parsing and validation failures.
+ */
 class PurlError extends Error {
   constructor(message?: string | undefined, options?: ErrorOptions) {
     super(formatPurlErrorMessage(message), options)

src/package-url.ts

@@ -54,6 +54,10 @@ const OTHER_SCHEME_PATTERN = /^[a-zA-Z][a-zA-Z0-9+.-]*:\/\//
 // Pattern to match purl-like strings with type/name format.
 const PURL_LIKE_PATTERN = /^[a-zA-Z0-9+.-]+\//
 
+/**
+ * Package URL parser and constructor implementing the PURL specification.
+ * Provides methods to parse, construct, and manipulate Package URLs with validation and normalization.
+ */
 class PackageURL {
   static Component = recursiveFreeze(PurlComponent)
   static KnownQualifierNames = recursiveFreeze(PurlQualifierNames)