1122: Add explicit type assist. r=matklad a=marcogroppo
This assist can be used to specify the explicit type in let statements. For example `let num = 1;` becomes `let num: i32 = 1;`.
The assist is applicable only if the inferred type is fully known.
Co-authored-by: Marco Groppo <marco.groppo@gmail.com>
cc284dad30