Compilers should be able to add keys to component's package.json

[itaymendel]

Description

Some compilers modify values in the package.json file. For example, ng-packagr needs to add the following keys:

"module": "...",
"es2015": "...",
"esm5": "...",
"esm2015": "...",
"fesm5": "...",
"fesm2015": "...",
"typings": "...",
"metadata": "..."

Describe the solution you'd like

Bit should provide an API for a compiler to add values to the package.json file.
There are two important things to note:

1. The compiler should modify the package.json during the build process.
2. When bit build runs and a compiler causes a modification to the package.json file - The component must not be modified.
3. There are several keys that a compiler must not be able to modify/create
   • name
   • version
   • main
   • dependencies
   • devDependencies
   • peerDependencies
   • license
   • bit

[davidfirst]

Implemented in #1769.
To add properties to the package.json, just add a new property packageJson to the return result.

In the example below, a compiler adds { "foo": "bar" } into the component package.json.

function compile(files, distPath) {
  const dists = files.map(f => doSomethingWhatever(f));
  return { dists, packageJson: { foo: 'bar' } };
}