Add return type annotations on Twig 3 to prepare Twig 4

[smnandre]

Started to add missing type annotation following @stof comment here.

Update: I checked here every class (but one) in the src directory.

But you should probably certainely have another look.

Remains:

• src/ExpressionParser.php, where i'm not confident enough about the best return types (AbstractExpression|null works almost everytime, but lack some precision)
• extra directory
• tests directory

I'm happy to let them to someone else.... this was less fun than I expected 😅

[smnandre]

(fabbot unrelated)

[smnandre]

(fixed my mistakes, all seems ok now)

[stof]

string|false actually. true is not a valid strategy.

[smnandre]

"true" will wait Twig4 / PHP 8.2 no ?

[stof]

I don't understand what you mean here. true would indeed require PHP 8.2 to be used. But the one we need here is false, not true

[smnandre]

Thank you very much @stof! (and sorry i make that many mistakes 😮‍💨 )

I've fixed all your comments

[fabpot]

Thank you @smnandre.