[Build] Remove tslib reference in es6 packages (First step towards esm)

[RaananW]

This is partly related to esm, but can be done externally and unrelated to the esm work.

In our es6 packages we still require tslib. tslib is used as a polyfill for missing functionality for the target. The issue is that we are using decorators, which is not available in any javascript version. It is currently in a stage-2 proposal (https://github.com/tc39/proposals#stage-2), after not even being requested for a long time. It also requires typescript to use the experimentalDeocators flag to have the feature enabled.

If we remove the use for decorators (or polyfill them ourselves similar to the way typescript does it), we can eliminate the need for a tslib reference which in turn will allow @babylonjs/core to be used directly in browser.