Updated babel/webpack docs to work with latest versions

[pereriksson]

Description

As a developer I tried setting up Mithril with Babel and Webpack but failed because of a variety of errors. After some research I have adjusted the documentation so that works with the latest versions.

Motivation and Context

Following the documentation does not lead to a working environment based on the latest versions. The guide needs to provide a working set of instructions.

How Has This Been Tested?

The updated instructions have been run in a clean environment and tested on a small Mithril/JSX codebase.
Babel 7.12.10
Webpack 5.11.1
Mithril 2.0.4

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/changelog.md