plugin: add proper support for appRoot

[vince-fugnitto]

Bug Description:

The vscode.env.appRoot is not properly implemented (likely stubbed initially) and should be updated:

theia/packages/plugin-ext/src/plugin/node/env-node-ext.ts

Lines 52 to 57 in d383fe2

	/**
	* Provides application root.
	*/
	get appRoot(): string {
	return __dirname;
	}

This means that that file's parent directory will be marked as the appRoot:

Steps to Reproduce:

1. include the following test extension (vscode-approot-0.0.1.zip)

2. start the application

3. execute the command Env: App Root