Add lint for excess trailing semicolons

[nathanwhit]

Closes #60876.
A caveat (not necessarily a negative, but something to consider) with this implementation is that excess semicolons after return/continue/break now also cause an 'unreachable statement' warning.

For the following example:

fn main() {
    extra_semis();
}
fn extra_semis() -> i32 {
    let mut sum = 0;;;
    for i in 0..10 {
        if i == 5 {
            continue;;
        } else if i == 9 {
            break;;
        } else {
            sum += i;;
        }
    }
    return sum;;
}

The output is:

warning: unnecessary trailing semicolons
 --> src/main.rs:5:21
  |
5 |     let mut sum = 0;;;
  |                     ^^ help: remove these semicolons
  |
  = note: `#[warn(redundant_semicolon)]` on by default

warning: unnecessary trailing semicolon
 --> src/main.rs:8:22
  |
8 |             continue;;
  |                      ^ help: remove this semicolon

warning: unnecessary trailing semicolon
  --> src/main.rs:10:19
   |
10 |             break;;
   |                   ^ help: remove this semicolon

warning: unnecessary trailing semicolon
  --> src/main.rs:12:22
   |
12 |             sum += i;;
   |                      ^ help: remove this semicolon

warning: unnecessary trailing semicolon
  --> src/main.rs:15:16
   |
15 |     return sum;;
   |                ^ help: remove this semicolon

warning: unreachable statement
 --> src/main.rs:8:22
  |
8 |             continue;;
  |                      ^
  |
  = note: `#[warn(unreachable_code)]` on by default

warning: unreachable statement
  --> src/main.rs:10:19
   |
10 |             break;;
   |                   ^

warning: unreachable statement
  --> src/main.rs:15:16
   |
15 |     return sum;;
   |                ^