Detect usage of npm or yarn in the project when asking to install more packages

[stof]

Currently, Encore always assumes yarn is used to add packages (with inconsistencies when the error does not come from Encore itself, see #57).

This causes confusion for users using npm instead of yarn (see #287).

When starting to work on Encore, yarn was considered superior to npm, because we compared it to version 3. With the version 5 of npm, the difference between both is smaller, so we should try to respect choices of the user.

Here is my proposal:

• if the project contains a yarn.lock file, display the yarn command to the user
• if the project contains a packages.lock file, display the npm command to the user
• otherwise, display the yarn command (following our existing rule where we favor yarn)

The last point may be changed in the future once geowarin/friendly-errors-webpack-plugin#33 gets resolved, to fix #57 (if they display the yarn version only when having a yarn.lock file, we may want to do the same, as running Yarn without lock file is not recommended anyway)

[stof]

Fixed in #291