Prototyping TS Server plugins on web

This is a first exploration for  microsoft#47376 microsoft/vscode#140455

It implements a very basic version of `require` for the web that lets us load plugins with a very specific structure. `require` being sync limits our options here, so I'm using `importScripts` plus a global to get the exports of a plugin. The plugin must also be compiled down to a single file at the moment

src/webServer/webServer.ts

@@ -1,4 +1,6 @@
 /*@internal*/
+/// <reference lib="dom" />
+
 namespace ts.server {
     export interface HostWithWriteMessage {
         writeMessage(s: any): void;
@@ -136,7 +138,34 @@ namespace ts.server {
             clearImmediate: handle => clearTimeout(handle),
             /* eslint-enable no-restricted-globals */
 
-            require: () => ({ module: undefined, error: new Error("Not implemented") }),
+            require: (initialPath: string, moduleName: string) => {
+                // TODO: figure out how to implement this without having to implement
+                // all of 'require'
+
+                // TODO: Path is currently hardcoded
+                const fullPath = initialPath + "/" + moduleName + "/lib/index.js";
+                if (fullPath.startsWith("http://") || fullPath.startsWith("https://")) {
+                    try {
+                        // Sync load the script into the current scope
+                        (globalThis as any).importScripts(fullPath);
+                    }
+                    catch (e) {
+                        return ({ module: undefined, error: new Error(`Could not evaluate module ${e}`) });
+                    }
+
+                    // Try to get export from global. Use the module name for this.
+                    // Not sure a better way to do this synchronously
+                    const module = (globalThis as any)[moduleName];
+                    if (module) {
+                        return ({ module, error: undefined });
+                    }
+                    else {
+                        return ({ module: undefined, error: new Error(`Could not find module export for ${moduleName}`) });
+                    }
+                }
+
+                return ({ module: undefined, error: new Error("Could not resolve module") });
+            },
             exit: notImplemented,
 
             // Debugging related