Guard against the custom package not being available by first checkin…

…g if the package is present and, then, loading it.

_extensions/webr/qwebr-document-engine-initialization.js

@@ -5,7 +5,7 @@ async function qwebrInstallRPackage(packageName) {
 
 // Function to load a single package
 async function qwebrLoadRPackage(packageName) {
-  await mainWebR.evalRVoid(`library(${packageName});`);
+  await mainWebR.evalRVoid(`if(requireNamespace('${packageName}', quietly = TRUE)) { library(${packageName}); }`);
 }
 
 // Generic function to process R packages