Conversion of Less to ES6 w/ TypeScript type linting support

[matthew-dean]

• Converted to ES2015 code using lebab, then replaced browserify with a Rollup build.
• TypeScript is added to the ESLint parser. The idea is that you can add types (in JSDoc) to verify that it's working. However, the transpiler is still Babel at the moment, so type violations won't throw errors (yet). There was concern some time ago about converting the codebase 100% to TypeScript, but I figured we could get the best of both worlds by keeping it technically JS, but getting compile-time type verification to catch potential errors.
• I removed the deprecated ieCompat option and the affected code. That was already false by default on 3.0+, but additionally, we weren't testing/supporting IE8 as a browser, so it no longer had any value.