Add Artisan command navigation (link, hover, completion, diagnostics)

[farpat]

This adds navigation for Artisan command names, like the extension already does for
config, route or controllerAction.

In Artisan::call('cache:clear'), the command name is now:

• a link: click on it to open the class of the command
• a hover: it shows the description of the command and its file
• a completion: it suggests all the commands of the project
• a diagnostic: it shows a warning if the command does not exist

The class is found with reflection, so commands from Laravel and from packages work too.

The name is detected in Artisan::call(), Artisan::queue(), Schedule::command() and
$schedule->command(). When arguments are added after the name, like
Schedule::command('queue:work --stop-when-empty'), only the name is used. It is the same
idea as middleware with parameters, like auth:api.

The parser does not need any change. The four options (Laravel.command.link, .hover,
.completion, .diagnostics) come from generatable.json, like the other features.