Transpile source during build to match target about langium

[spoenemann]

Bundling can be useful for the VS Code extension. It is not necessary for library code.

[danieldietrich]

microbundle takes babelrc into account, see developit/microbundle#25

[danieldietrich]

I think currently there isn't a specific requirement to bundle the code because we develop a library, not an applications. As a library, langium will be included in other applications, which on their turn will target a specific environment (node or browser).

Therefore I will remove the bundler and use tsc for now to emit .js and .d.ts files.

[danieldietrich]

For the VS Code extensions it will be simpler because source and target are currently set to ES6 (default of the yeoman template) - no polyfills needed.