wrap service provider boot code in telescope.enabled conditional

[mattquest]

Currently, disabling telescope through the TELESCOPE_ENABLED environment variable will prevent Telescope::start from running but does not prevent telescope from registering routes, migrations, views etc. in the TelescopeServiceProvider->boot method.

This can cause unexpected errors and problems in situations like a testing environment where telescope is disabled and the migrations are not expected to be registered.

[Namoshek]

This PR actually broke some Dusk tests of mine because the Telescope route of a link in the menu was not registered.

Disabling a package entirely seems very drastic to me. I would suggest reverting this and letting affected users registering the TelescopeServiceProvider only in non-testing environments:

// in AppServiceProvider::register()
if ($this->app->environment('testing') === false) {
    $this->app->register(\Laravel\Telescope\TelescopeServiceProvider::class);
}