chore: Copied typings from DefinitelyTyped

Adds TypeScript as a dev dependency and specifies `index.d.ts` as typings, with `--noEmit` tests run as part of `test`.

Author: Josh Goldberg

Makefile

@@ -3,7 +3,7 @@
 # Tests
 # 
 
-test: test-node
+test: test-node test-typescript
 
 test-node: 
 	@printf "\n  ==> [Node.js]\n"
@@ -14,6 +14,9 @@ test-browser:
 	@make build
 	@printf "\n\n  Open 'test/index.html' in your browser to test.\n\n"
 
+test-typescript:
+	@./node_modules/.bin/tsc test/typings.ts index.d.ts --noEmit
+
 #
 # Components
 # 

index.d.ts

@@ -0,0 +1,11 @@
+type AssertionError<T = {}> = Error & T & {
+    showDiff: boolean;
+};
+
+interface AssertionErrorConstructor {
+    new<T = {}>(message: string, props?: T, ssf?: Function): AssertionError<T>;
+}
+
+declare const AssertionError: AssertionErrorConstructor;
+
+export = AssertionError;