Give a useful example of `let` patterns

[jariji]

@match 1 begin
    let x = 1 end => x
end

is the only example, which doesn't show why I would want to use it.

[Taine Zhao]

1. Creating (local) bindings during matching is a function that cannot be expressed by other syntax.

2. In terms of the real-world use case, sometimes you might want to use one body for multiple cases, then some cases need to create default values:

   f(expr) = @match expr begin
         :($f($(args...); $(kwargs...))) ||
         :($f($(args...))) && let kwargs = [] end =>
             (f, args, kwargs)
   end
   
    julia> f(:(func(a, b; c= 1)))
    (:func, Any[:a, :b], Any[:($(Expr(:kw, :c, 1)))])
   
    julia> f(:(1+2))
    (:+, Any[1, 2], Any[])