Accept yarn instead of npm

[ggreif]

Currently npm is hard-coded in dfx:

$ git grep '"npm"'
src/dfx/src/commands/new.rs:    std::process::Command::new("npm")
src/dfx/src/commands/new.rs:    std::process::Command::new("npm")
src/dfx/src/lib/builders/assets.rs:        let mut cmd = std::process::Command::new("npm");

I am asking for support for the (more-or-less) drop-in replacement yarn. I had some nice success using it in connection with vuetify:

• git clone https://github.com/nop33/dfinity-vue -b vuetify
• dfx start --background --clean
• yarn add webpack-cli
• env NPM=yarn dfx deploy (suggested way of using yarn instead of npm)
• open http://localhost:8000/?canisterId=ryjl3-tyaaa-aaaaa-aaaba-cai

The assets.rs invocation works out of the box (this is what the above commands trigger). The new.rs would need some adaptation (maybe probing).

An alternative would be to introduce a --toolchain argument (in-lieu-of NPM env-var): dfx --toolchain=yarn would just select the yarn incarnation of the commands and could potentially also do the yarn add webpack-cli step.

I am happy to send a PR if you think this is a good idea.

[sesi200]

Also referenced in #1584