feat: improvements for type checker (current limitations)

[leonidas1712]

• Programs like let x = {}; , let x = { 2; } where assigned to blk with no last expr, should either be rejected or accepted but runtime must change to allow this (assignment to unit is allowed in Rust)
• Currently if blk decls have any errors, those are thrown first without checking last expr because last expr might depend on decls.
  e.g

let x : int = true;
let y = 20;
{
    let y : bool = 20;
}
x + false

x+false is the last expr, add is not allowed for bool but type checker won't throw this until prev errors are fixed.

• Need more advanced control flow analysis to see what the last expr depends on (probably after 4215 over)