For global variable declaration, suggest ```const``` if ```let``` is used instead

[Rageking8]

Given the following code : link

let X: i32 = 12;

fn main() {

}

The current output is:

   Compiling playground v0.0.1 (/playground)
error: expected item, found keyword `let`
 --> src/main.rs:1:1
  |
1 | let X: i32 = 12;
  | ^^^ expected item

error: could not compile `playground` due to previous error

Ideally the output should inform the user that let cannot be used to create global variables and suggest using const instead. (Might need a little rephrasing).

[chenyukang]

@rustbot claim

[Rageking8]

Completed by #100396