Support parsing plans

[glennsarti]

Migrated from puppetlabs/puppet-vscode#224

---

When parsing Puppet plans, puppet requires the --tasks flag (or Puppet[:tasks]) set to true.

Currently this extension errors on plans. Presumably that could be fixed by setting the tasks setting to true when evaluating files in a plans directory of a module.

As an example

plan foo::pattern(
  Pattern[/^[^\/\\]*$/] $p = 'hello'
) {
  notice($p)
}

results in the error Could not parse for environment *root*: Syntax error at 'p' (line: 2, column: 25).

Using macOS 10.13, Puppet 5.4.0, VSCode 1.20.1.