Add TypeScript support

[manovotny]

Will likely use @typescript-eslint/eslint-plugin.

A running list of things to consider too.

eslint/no-shadow

Disabled because of @typescript-eslint/no-shadow, if we choose to use it.

eslint/no-unused-expressions

Disabled because of @typescript-eslint/no-unused-expressions, if we choose to use it.

eslint/no-use-before-define

Disabled because of @typescript-eslint/no-use-before-define, if we choose to use it.

eslint/require-await

Disabled because of @typescript-eslint/require-await`, if we choose to use it.

eslint/spaced-comment

Settings needs to be changed to allow for TypeScript's triple-slash directives.

'spaced-comment': [
    'error',
    'always',
    {
        markers: ['/'],
    },
],

react/jsx-filename-extension

Settings need to be changed to allow TypeScript files.

Just need to enhance the existing rule in index.js.

if (usesTypeScript) {
    config.rules['react/jsx-filename-extension'][1].extensions.push('.tsx');
}

[manovotny]

This will fixed with the v7 release.