editor/code: Enable noUncheckedIndexedAccess ts option

https://www.typescriptlang.org/tsconfig#noUncheckedIndexedAccess

editors/code/src/nullable.ts

@@ -0,0 +1,19 @@
+export type NotNull<T> = T extends null ? never : T;
+
+export type Nullable<T> = T | null;
+
+function isNotNull<T>(input: Nullable<T>): input is NotNull<T> {
+    return input !== null;
+}
+
+function expectNotNull<T>(input: Nullable<T>, msg: string): NotNull<T> {
+    if (isNotNull(input)) {
+        return input;
+    }
+
+    throw new TypeError(msg);
+}
+
+export function unwrapNullable<T>(input: Nullable<T>): NotNull<T> {
+    return expectNotNull(input, `unwrapping \`null\``);
+}