[5.7] Tuple notation for UrlGenerator actions

[sebastiandedeyne]

This PR allows you to use a tuple notation for generating action urls. This is especially useful when importing a controller class.

action([FooController::class, 'bar']);

I think it'd make sense to do this in the router too, but that's a but trickier because we'd need to detect if it's an associative array (for ['uses' => '...']) first.

[sisve]

Do we have anything called "tuple notation" in the framework already? This looks like php's builtin callable syntax.

[devcircus]

Seems like this callable syntax was PRd for routes a few weeks ago.

[taylorotwell]

Send a docs PR to master branch if you don't mind.

[sebastiandedeyne]

@sisve The exact name doesn't really matter since it doesn't exist in the code, just this PR title :) "Callable syntax" makes more sense though, if it ends up in the docs I'll write it like that.

@devcircus Thanks for mentioning, will hold off on that then.

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Added to my todo list, I'll send a docs PR later this week 👍