Jest collect coverage

- Enabled Jest to collect coverage.
- Set ESLint & Git to ignore folder `coverage`.

Author: iamWing

.eslintignore

@@ -1,5 +1,6 @@
 node_modules
 dist
+coverage
 out
 public
 jest.config.js

.gitignore

@@ -1,4 +1,5 @@
 .vscode/
+coverage/
 dist/
 node_modules/
 out/

jest.config.js

@@ -25,5 +25,6 @@ module.exports = config = {
   testMatch: [
     '**/tests/**/*.(spec|test).(ts?(x)|js?(x))',
   ],
+  collectCoverage: true,
   verbose: true,
 };