Rollup merge of #88553 - theo-lw:issue-88276, r=estebank

Improve diagnostics for unary plus operators (#88276)

This pull request improves the diagnostics emitted on parsing a unary plus operator. See #88276.

Before:

```
error: expected expression, found `+`
 --> src/main.rs:2:13
  |
2 |     let x = +1;
  |             ^ expected expression
```

After:

```
error: leading `+` is not supported
 --> main.rs:2:13
  |
2 |     let x = +1;
  |             ^
  |             |
  |             unexpected `+`
  |             help: try removing the `+`
```

compiler/rustc_ast/src/token.rs

@@ -586,6 +586,13 @@ impl Token {
         self.is_non_raw_ident_where(|id| id.name.is_bool_lit())
     }
 
+    pub fn is_numeric_lit(&self) -> bool {
+        matches!(
+            self.kind,
+            Literal(Lit { kind: LitKind::Integer, .. }) | Literal(Lit { kind: LitKind::Float, .. })
+        )
+    }
+
     /// Returns `true` if the token is a non-raw identifier for which `pred` holds.
     pub fn is_non_raw_ident_where(&self, pred: impl FnOnce(Ident) -> bool) -> bool {
         match self.ident() {