Rollup merge of #144733 - Muscraft:secondary-file-sigil, r=compiler-errors

fix: Match width of ascii and unicode secondary file start

The current [unicode secondary file start](64ca23b623/compiler/rustc_errors/src/emitter.rs (L2991)) is only three characters, whereas the ASCII variant and normal [file start](64ca23b623/compiler/rustc_errors/src/emitter.rs (L2983-L2984)) are four characters. This slight difference caused the paths following a Unicode secondary file start to not align with other structured elements.

compiler/rustc_errors/src/emitter.rs

@@ -2988,7 +2988,7 @@ impl HumanEmitter {
     fn secondary_file_start(&self) -> &'static str {
         match self.theme {
             OutputTheme::Ascii => "::: ",
-            OutputTheme::Unicode => " ⸬ ",
+            OutputTheme::Unicode => " ⸬  ",
         }
     }