Rollup merge of #91476 - m-ou-se:ferris-identifier, r=estebank

Improve 'cannot contain emoji' error.

Before:

```
error: identifiers cannot contain emoji: `🦀`
 --> src/main.rs:2:9
  |
2 |     let 🦀 = 1;
  |         ^^
```

After:
```
error: Ferris cannot be used as an identifier
 --> src/main.rs:2:9
  |
2 |     let 🦀 = 1;
  |         ^^ help: try using their name instead: `ferris`
```

r? `@estebank`

compiler/rustc_span/src/symbol.rs

@@ -630,6 +630,7 @@ symbols! {
         fdiv_fast,
         feature,
         fence,
+        ferris: "🦀",
         fetch_update,
         ffi,
         ffi_const,