refactor: improve Node.js 18 deprecation warning check

- Extract deprecation warning logic into shouldShowDeprecationWarning function
- Replace startsWith check with regex pattern for more accurate version matching
- Add explicit undefined check for process.version

Author: grdsdev

src/index.ts

@@ -41,12 +41,16 @@ export const createClient = <
 }
 
 // Check for Node.js 18 deprecation
-if (
-  typeof window === 'undefined' &&
-  typeof process !== 'undefined' &&
-  process.version &&
-  process.version.startsWith('v18')
-) {
+function shouldShowDeprecationWarning(): boolean {
+  return (
+    typeof window === 'undefined' &&
+    typeof process !== 'undefined' &&
+    process.version !== undefined &&
+    /^v18\./.test(process.version)
+  )
+}
+
+if (shouldShowDeprecationWarning()) {
   console.warn(
     `⚠️  Node.js 18 is deprecated and will no longer be supported in future versions of @supabase/supabase-js. ` +
       `Please upgrade to Node.js 20 or later. ` +