Avoid need for end-user to install Vale CLI

[apupier]

2 solutions:

• Embed the CLI:
  ** It will avoid to ask user where it is installed.
  ** It is possible to provide platform-specific VS code extension. But as a first iteration given that the vale cli size is not that important (<7Mo), we can imagine to embed all of them in the extension.
  ** the drawback is that it requires to release VS code when a new release of vale cli is provided
• Download CLI automatically when vale cli is onot available on classpath or provided through the settings
  ** ensures latest version of cli used
  ** harder to implement
  ** not working on airgapped environments
  ** if latest cli contains incompatible change, it will break the vs code extension