Parser: allow anonymous block args

[asterite]

Related to #7157

In addition to being able to write this:

def foo(x, y, &block)
end

Now you can write it like this:

def foo(x, y, &)
end

That is, you can omit the block name.

Of course it also works with a type signature:

def foo(x, y, & : Int32 -> Int32)
end

It might look more obscure but there's a reason we might go this way.

Why?

Right now the compiler infers a few things based on the signature and method definition:

• If a method has yield inside it the compiler knows it receives a block
• If a method mentions the block argument then the compiler considers the block is captured and it will form a closure over it when invoked

This is nice but it has a few disadvantages:

• because you can just use yield and omit the block argument it's not immediately clear from the method signature whether a method accepts a block or not
• you must use &block if you want to say you receive a block, even if you don't plan to yield (Nil#try does this) so it's a bit asymmetric that sometimes you need to write it and sometimes not
• if yield is generated through a macro then you are again forced to specify the block argument
• if the block argument is mentioned inside a macro then the compiler doesn't know it actually forms a closure (Block argument isn't seen inside macro code #2992) and one has to mention the block argument somewhere outside the macro code as an ugly workaround

So the idea would be:

• Just & means a non-captured block argument: because it doesn't have a name there's no way to capture it
• &block means a captured block argument, and the compiler will verify that it's used in the semantic phase (because it might be mentioned inside a macro)
• &block without a mention will give a compile error suggesting to either change it to & to avoid a closure or to mention it somehow
• if you yield and the method signature doesn't have & the compiler will tell you

Now, this PR is just the beginning direction: there's no breaking change here, just the possibility to omit the block argument name.