Add typeof keyword

[albeva]

Support deducing type from a given expression. Similar to C++ decltype

Example:

rem legal:
dim a as typeof(AnotherType)
dim b as typeof(func) ptr
dim c as typeof(a + b)
dim d as typeof(3.1415)
dim e as typeof(not true)
dim f as typeof(null)
dim g as typeof(typeof(foo) ptr)
dim h = bar as typeof(moo)

rem illegal:
rem type foo as typeof(s.t)
rem declare sub foo(foo as typeof(x))

Grammer:

TypeOf = "typeof" "(" (Expr | TypeExpr) ")" ;

Limitations

Because types and functions can be declared out of order and invoked without forward declarations - it is far too complex to allow type aliases or function parameters / return type to be deduced from typeof() expression. So for now implement only for variable declarations.

The problem is with indirect type deductions. Because currently type declarations are parsed first and only then regular statements, it is far too complex to resolve this in a reasonably simple way:

dim s as MyFoo          rem TypeProxy(unknown MyFoo)
s.hello = "Hello"       rem type must be realized. But `f` is still unknown.

var f = Foo()           rem to know type of `f` expression must solved
type MyFoo as typeof(f) rem TypeProxy(expression (f))

type Foo
    hello as zstring
end type