Allow predeclaration of local functions

[catwell]

Consider this code sample:

local foo : function()
function foo() end
foo()

Before this change it would fail with:

========================================
1 error:
invalid.tl:2:1: functions need an explicit 'local' or 'global' annotation

This change makes it valid again (it was the case before 0.14).

Rationale: in Lua function foo() end is sugar for foo = function() end, and I suppose it should remain the same in Teal.

This kind of code is useful to write mutually recursive functions such as this:

local collatz_odd : function(integer): integer

local function collatz_even(n: integer): integer
    print(n)
    n = n // 2
    if n % 2 == 1 then
        return collatz_odd(n)
    end
    return collatz_even(n)
end

function collatz_odd(n: integer): integer
    print(n)
    if n == 1 then
        return 1
    end
    return collatz_even(3 * n + 1)
end

collatz_odd(7)

[github-actions[bot]]

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[hishamhm]

I had no strong feelings myself about supporting or not this Lua idiom, but I suppose it is consistent and unambiguous enough, and given the finished PR, I'm happy to merge it :)