Add typescript support for storefront javascript

INFO

This document represents an architecture decision record (ADR) and has been mirrored from the ADR section in our Shopware 6 repository. You can find the original version here

Context

• We want to add TypeScript support to the Storefront, to make use of all of it's features increasing the overall developer experience, quality and maintainability.
• The main concern is the compatibility to existing Storefront plugins, which have been built in previous versions without TypeScript support.
• TypeScript files need to be compatible with JavaScript files and vice versa, for both the Storefront internally, and also for plugins.

Decision

• To prevent any breaks in our current Storefront stack, we will add TypeScript language support to the current babel chain, using the preset @babel/preset-typescript.
• To prevent any breaks for existing Storefront plugins, we won't replace any publicly used .js files with .ts files, without proper deprecation.

Consequences

• TypeScript (.ts and .tsx) files are now supported by the Storefront.
• Storefront plugins can now be developed using TypeScript and the actual Storefront JavaScript can be incrementally converted to .ts files from now on.
• TypeScript files and JavaScript files are compatible and can be imported to each other.