Move on to a TypeScript project

[fthouraud]

I propose to transform Veggies to a TypeScript module.

This move should strengthen our code while making it simpler to maintain.

There will be several steps:

• Initialise the TypeScript project and update related components (ESLint, for instance),
• Migrate all sources to TypeScript (source code as well as file extension),
• Update our actions to build TS sources then package the transpiled sources (with declaration files).

This is huge work but that should improve the code's quality and bring a good amount of comfort.