EDITOR: Enable strict typechecking of typescript. (#1493)

* EDITOR: Tweak typescript language server configuration to match use.

- We allow importing files with `.ts`/`.tsx` extensions.
- We use an file-at-a-time transpiler, so we don't support features
  that require understanding the full type system.
- We use the classic `React.createElement` transform.

src/ScriptEditor/ScriptEditor.ts

@@ -63,26 +63,45 @@ export class ScriptEditor {
 
     // Add ts definitions for API
     const source = netscriptDefinitions.replace(/export /g, "");
-    for (const languageDefaults of [
+    for (const [language, languageDefaults] of [
-      monaco.languages.typescript.javascriptDefaults,
+      ["javascript", monaco.languages.typescript.javascriptDefaults],
-      monaco.languages.typescript.typescriptDefaults,
+      ["typescript", monaco.languages.typescript.typescriptDefaults],
-    ]) {
+    ] as const) {
       languageDefaults.addExtraLib(source, "netscript.d.ts");
       languageDefaults.addExtraLib(reactTypes, "react.d.ts");
       languageDefaults.addExtraLib(reactDomTypes, "react-dom.d.ts");
+      languageDefaults.setCompilerOptions({
+        ...languageDefaults.getCompilerOptions(),
+        // We allow direct importing of `.ts`/`.tsx` files, so tell the typescript language server that.
+        allowImportingTsExtensions: true,
+        // We use file-at-a-time transpiler. See https://www.typescriptlang.org/tsconfig/#isolatedModules
+        isolatedModules: true,
+        // We use the classic (i.e. `React.createElement`:) react runtime.
+        jsx: monaco.languages.typescript.JsxEmit.React,
+        // We define `React` and `ReactDOM` as globals. Don't mark using them as errors.
+        allowUmdGlobalAccess: true,
+        // Enable strict typechecking.
+        // Note that checking in javascript is disabled by default but can be enabled via `// @ts-check`.
+        // This enables strictNullChecks, which impacts reported types, even in javascript.
+        strict: true,
+        noImplicitAny: language === "typescript",
+        noImplicitReturns: true,
+      });
+      languageDefaults.setDiagnosticsOptions({
+        ...languageDefaults.getDiagnosticsOptions(),
+        // Show semantic errors, even in javascript.
+        // Note that this will only happen if checking is enabled in javascript (e.g. by `// @ts-check`)
+        noSemanticValidation: false,
+        // Ignore these errors in the editor:
+        diagnosticCodesToIgnore: [
+          // We define `React` and `ReactDOM` as globals. Don't mark using them as errors.
+          // Even though we set allowUmdGlobalAccess, it still shows a warning (instead of an error).
+          // - 'React' refers to a UMD global, but the current file is a module. Consider adding an import instead.(2686)
+          2686,
+        ],
+      });
     }
-    monaco.languages.typescript.typescriptDefaults.setCompilerOptions({
+
-      ...monaco.languages.typescript.typescriptDefaults.getCompilerOptions(),
-      jsx: monaco.languages.typescript.JsxEmit.ReactJSX,
-      allowUmdGlobalAccess: true,
-    });
-    /**
-     * Ignore these errors in the editor:
-     * - Cannot find module ''. Did you mean to set the 'moduleResolution' option to 'nodenext', or to add aliases to the 'paths' option?(2792)
-     */
-    monaco.languages.typescript.typescriptDefaults.setDiagnosticsOptions({
-      diagnosticCodesToIgnore: [2792],
-    });
     monaco.languages.json.jsonDefaults.setModeConfiguration({
       ...monaco.languages.json.jsonDefaults.modeConfiguration,
       //completion should be disabled because the