Install instructions don't take package-manager setting into account

[canadaduane]

When running chomp without auto-install, it provides warning messages showing how to install the necessary dependencies, e.g.:

Chomp: Some packages are missing. Please run npm install @jspm/generator mkdirp -D

I'm using pnpm and (stupidly) copy/pasted the npm command. Would be nice if it took package-manager into account, i.e.:

Chomp: Some packages are missing. Please run pnpm install @jspm/generator mkdirp -D

[guybedford]

Do you feel like making a PR? The template line is here - https://github.com/guybedford/chomp/blob/main/templates/npm.toml#L31. It should just be a substitution like the others. That it's JS in a toml string outputting JSON that will be interpreted as task TOML gets a bit confusing, but makes sense with enough staring :)

[guybedford]

*A string in a shell command on a JSON task output from a JS function that's a string in a TOML file :P