Fix rustup and standardize on yarn - Gitpod fixes

[mdp]

Fixes #11

rustup toolchain install stable fails on Gitpod and then messes up the entire rustup install, remove and just install the wasm32 toolchain and it works fine.

Using yarn and npm are problematic:

1. Currently 'yarn' is run from v16 of node, even though nvm is set to v12 - ensure yarn is installed on v12
2. yarn is used to install some dependencies, then npm for the rest - standardize on yarn.
3. Use yarn global to install near-cli