Type declaration of no-existing variables

[PalumboN]

Validate that all type declarations are linked to a variable available in the scope. For example, to avoid cases like

foo
    <var: #x type: #int>
    | theX |

This is useful to eagerly detect issues (for example, a renamed variable but with a not renamed type declaration) before going to the entire slang->c->native compilation process that is long.

• [x] Subissue #603
• [ ] Subissue #598
• [ ] Subissue #599