Implement `as <type>` declaration

We need as <type> declarations for the compiler (if we are smarter) and also for the runtime. This should be a

var x: integer | text = 42
// x is integer | text

var y = x as integer // x is asserted to be integer and typeof y is integer
var y = x as text // x is asserted to be text (invalid, throws)

@example :: (1 + 2)

Compiler has to inject a assertion such as

@example :: (1 + 2) as integer

if we can not analyze the remote code (due to missing types), it will be treated as "unknown" and we have to cast/match to use for further processing (such as apply chains)

• Implement in grammar
• Implement in compiler
• Implement in runtime