refactor: replace string-matching error checks with custom error classes

src/root.ts

@@ -3,6 +3,7 @@ import * as path from "node:path";
 import { pathToFileURL, fileURLToPath } from "node:url";
 import { servers } from "../server.ts";
 import type { ServerConfig } from "./types.ts";
+import { UnsupportedExtensionError } from "./types.ts";
 
 // Resolve File URI To Local Path
 export function uriToPath(uri: string): string {
@@ -20,7 +21,7 @@ export function pickServer(filePath: string): ServerConfig {
   const ext = path.extname(filePath).replace(/^\./, "");
   const hit = servers.find((s) => s.match.includes(ext));
   if (!hit) {
-    throw new Error(`No LSP server registered for extension ".${ext}"`);
+    throw new UnsupportedExtensionError(`.${ext}`);
   }
   return hit;
 }