Vscode-eslint: Add support for `pnpm`

[ExE-Boss]

pnpm is the third largest node package manager, which uses symlinks, hardlinks and isolated trees, which prevents packages from using packages which they haven’t declared in their dependencies.

I believe that as it’s the third largest, it should also be supported in the eslint.packageManager configuration key.

See also:

• https://www.kochan.io/nodejs/why-should-we-use-pnpm.html
• https://medium.com/pnpm/flat-node-modules-is-not-the-only-way-d2e40f7296a3

/cc @zkochan

[dbaeumer]

@ExE-Boss would you be willing to contribute this. I have no expierences with pnpm and I am sure things will again be different.

[dbaeumer]

Closing. See https://github.com/Microsoft/vscode-eslint/pull/572