Report validation errors

[kaloyan-raev]

The language server should report any validation errors found by the PHP parser back to the client.

This should be done via PublishDiagnostics Notifications.

The PHP Parser can be run in error recovery mode to avoid stopping on the first error found. Thus providing better user experience.

The validation can be triggered on incoming notifications from the client like:

• DidOpenTextDocument Notification
• DidChangeTextDocument Notification
• DidChangeWatchedFiles Notification