Use standard TypeScript configuration (#27)

The TypeScript configuration has been updated to use the same settings
we use in the MetaMask module template. The changes are:

* Enable inline source maps.
* Target ES2017 rather than ES5.
* Allow the use of ES2020 syntax/APIs and DOM APIs in the TypeScript source.
* Explicitly declare the root directory, to prevent it from changing
accidentally.
* Remove unnecessary type roots and excludes (both included the non-
  existent `types` directory, and were set to default aside from that).
* Enable `esModuleInterop` for better CommonJS compatibility.

tsconfig.json

@@ -1,14 +1,16 @@
 {
   "compilerOptions": {
-    "moduleResolution": "Node",
     "declaration": true,
+    "esModuleInterop": true,
+    "inlineSources": true,
+    "lib": ["DOM", "ES2020"],
     "module": "CommonJS",
+    "moduleResolution": "Node",
+    "outDir": "dist",
+    "rootDir": "src",
     "sourceMap": true,
     "strict": true,
-    "target": "es5",
-    "typeRoots": ["./types", "./node_modules/@types"],
-    "outDir": "dist"
+    "target": "ES2017"
   },
-  "include": ["src/**/*.ts"],
-  "exclude": ["node_modules", "types"]
+  "include": ["src/**/*.ts"]
 }