Add Psalm Language Server support

[lhpalacio]

This PR adds support for Psalm, a static analysis tool for PHP.

Enable Psalm

Add to your settings.json:

{
  "languages": {
    "PHP": {
      "language_servers": ["psalm"]
    }
  }
}

Custom Path (Optional)

{
  "lsp": {
    "psalm": {
      "initialization_options": {
        "command": ["vendor/bin/psalm-language-server"]
      }
    }
  }
}

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[lhpalacio]

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[cla-bot]

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