[api-minor] Include the /Lang-property in the documentInfo, and use it in the viewer (issue 14110)

*Please note:* This is a tentative patch, since I don't have the necessary a11y-software to actually test it.

To avoid having to add a new API-method just for a single string, I figured that adding the new property to the existing `documentInfo`-data (accessed via `PDFDocumentProxy.getMetadata` in the API) will hopefully be deemed acceptable.

src/core/catalog.js

@@ -89,6 +89,15 @@ class Catalog {
     );
   }
 
+  get lang() {
+    const lang = this._catDict.get("Lang");
+    return shadow(
+      this,
+      "lang",
+      typeof lang === "string" ? stringToPDFString(lang) : null
+    );
+  }
+
   /**
    * @type {boolean} `true` for pure XFA documents,
    *   `false` for XFA Foreground documents.

src/core/document.js

@@ -1160,6 +1160,7 @@ class PDFDocument {
 
     const docInfo = {
       PDFFormatVersion: version,
+      Language: this.catalog.lang,
       IsLinearized: !!this.linearization,
       IsAcroFormPresent: this.formInfo.hasAcroForm,
       IsXFAPresent: this.formInfo.hasXfa,