Implement local node.js support via nvm

[pjeby]

This would be done by .envrc checking for .deps/.nvm and sourcing the relevant bits with NVM_DIR set appropriately; there should probably be a --no-use option set by default. PATH should include node_modules/bin. The nvm module itself would just make sure that .deps/.nvm exists, and gets sourced.

Alternately, it might be simpler to use nvm only in a subshell for installation purposes; that is, to just link a specific nvm-supplied version to .deps/bin. That would avoid path manipulation issues, probably. The nvm module could supply an nvm function that calls the real nvm function in a subshell and symlinks the tools from the bin dir where node is. (e.g. via nvm which).

This does present the issue of whether to use a yarn or npm install if there's a package.json present; that should probably be done via a module specifically for that purpose, distinct from the nvm module. One might wish to use nvm and node without those behaviors or indeed a package.json at all.

[pjeby]

This was basically implemented via .nvmrc support last year, but needs to be documented.