chore: update tsconfig-client.json to use ESM

[layershifter]

This PR updates tsconfig-client.json to target module: es6 to enable tree-shaking properly, this also mentioned in official Webpack guide (https://webpack.js.org/guides/typescript/).

module: commonjs

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
const react_northstar_1 = require("@fluentui/react-northstar");
console.log(react_northstar_1.teamsTheme);

Output is 1.37 MiB because Module Concatenation used for tree shaking in Webpack 4 does not work with Common JS.

module: es6

import { teamsTheme } from "@fluentui/react-northstar";
console.log(teamsTheme);

Output is 417 KiB.