Enable the strict typechecking

[AndreyBelym]

Try to implement transpiling in #16 and enable the strict typechecking via the strict: true in another PR.

[AndreyBelym]

Note: to enabling linting TypeScript code, you'll need to add @typescript-eslint/eslint-plugin & @typescript-eslint/parser to your project and adjust .eslintrc:

{
  "parser": "@typescript-eslint/parser",
  "parserOptions": {
    "sourceType": "module"
  },
  "extends": [
    "eslint:recommended",
    "plugin:@typescript-eslint/recommended"
  ],
  // NOTE: remove original ESLint rules that will be covered by typescript-eslint rules
  "rules": {
     "@typescript-eslint/no-extra-parens": 2,
     "@typescript-eslint/no-use-before-define": [2, "nofunc"],
     "@typescript-eslint/no-var-requires": [0],
     "@typescript-eslint/explicit-function-return-type": [2, { "allowExpressions": true }],
     "@typescript-eslint/no-unused-vars": 2
  }
}