Add ES-module support and improve package building

[szimeth-jozef]

I ran into an obstacle described in this issue #66, so I came up with a solution. I implemented bundling for multiple module types

• Added ES-modules
• Kept AMD modules (for backward compatibility reasons and I noticed that azure-devops-extension-api depends on this package/library)
• Bundled IIFE for browser use

by replacing the build script with rollup module bundler.

Other changes:

• Upgraded typescript version
• Added missing types
• Repalced vars with const/let to improve code consistency

[ChronosMasterOfAllTime]

It seems we have 3 PR's targeting the same issue:

74

56

I would vote for this PR since it generates support for multiple module types.