Adds detection for yarn, and '--no-yarn' to suppress detection

[rbuckton]

This adds automatic detection of yarn based on the presence of either a yarn.lock or .yarnrc file in the current directory. Passing --yarn or --no-yarn disables automatic detection.

Fixes #480

[felipecrs]

LGTM

[rbuckton]

This might need to be updated if #458 goes in, as this currently only looks for yarn.lock and .yarnrc in the current directory, but if you're publishing an from a package subdirectory in a yarn workspace, neither of those two files will be present since the current directory is not the workspace root.

[rbuckton]

Build is failing before it starts due to the node version used to run the build:

yarn install v1.22.5
[1/5] Validating package.json...
[2/5] Resolving packages...
[3/5] Fetching packages...
error [email protected]: The engine "node" is incompatible with this module. Expected version ">=10". Got "8.17.0"
error Found incompatible module.
info Visit https://yarnpkg.com/en/docs/cli/install for documentation about this command.

https://dev.azure.com/vscode/VSCE/_build/results?buildId=53730&view=logs&j=12f1170f-54f2-53f3-20dd-22fc7dff55f9&t=f8ed7bd8-2a7f-56f6-9385-7fc29a8b5b7b&l=18

[rbuckton]

@joaomoreno I bumped the node version in azure-pipelines.yml to get the build to pass. I can revert that if you still want to be sure it builds/tests on NodeJS 8, but I can't do much about husky 4.3.0 not supporting node 8.

[joaomoreno]

Thanks!