Don't check for yarn.lock

[hediet]

Can we remove this check:

vscode-vsce/src/npm.ts

Line 188 in 76b06f2

if (await exists(path.join(cwd, 'yarn.lock'))) {

And rely on yarn to throw error No lockfile in this directory. Run yarn install to generate one.?
This makes spotting mistakes easier. When using --yarn, the user very much expects the tool to be in a yarn working dir.

Also, this check doesn't work for yarn workspaces (though much other stuff also doesn't work for yarn workspaces, but they can be hacked around - this check cannot be hacked around).

[joaomoreno]

Makes sense, wanna send over a PR?