Rollup merge of rust-lang#103104 - SUPERCILEX:sep-ref, r=dtolnay

Stabilize `main_separator_str`

See reasoning here: rust-lang#94071 (comment). Closes rust-lang#94071.

Author: Yuki Okushi

library/std/src/path.rs

@@ -271,7 +271,7 @@ pub const MAIN_SEPARATOR: char = crate::sys::path::MAIN_SEP;
 /// The primary separator of path components for the current platform.
 ///
 /// For example, `/` on Unix and `\` on Windows.
-#[unstable(feature = "main_separator_str", issue = "94071")]
+#[stable(feature = "main_separator_str", since = "CURRENT_RUSTC_VERSION")]
 pub const MAIN_SEPARATOR_STR: &str = crate::sys::path::MAIN_SEP_STR;
 
 ////////////////////////////////////////////////////////////////////////////////