Cat: QoL for expanded mode

[Sainan]

A simple metatable like this would be pretty unintrusive (= invisible to dumpvar & for loops) and would massively simplify usage of CaT in expanded mode:

local { cat } = require "*"

local cat_mt = {
    __index = function(data, key)
        for data as node do
            if node.name == key then
                return node
            end
        end
        return nil
    end
}

local data = cat.decode([[
Foo: Foobar
Bar: Barfoo
]], "expanded")
setmetatable(data, cat_mt)

print(data["Foo"].value) --> Foobar

I'd say once this is addressed, expanded mode could be the default (as per #733).